Now, I've studied human proportions extensively over the past month, not only arm and leg proportions, but other body part proportions too, i.e. hands, fingers, chest, etc.
If we assume normally proportioned individuals (no outliers), his / her arm length (from acromion bone down to middle finger) should be his / her height divided by 2.3. Let's call this total arm length or TAL.
Now, TAL includes hand and fingers and it makes harder to exactly gauge how long should one's actual arm length (from acromion bone down to radial bump) be. Again, for normally proportioned individuals (no outliers) it should be 0.33 of total body height. Let's call this actual arm length or AAL.
Now, let's compare 2 normally proportioned individuals, one at 165 cm tall and one at 180 cm tall.
165 cm man's stats:
TAL = 165 / 2.3 ~ 71.74 cm
AAL = 165 x 0.33 ~ 54.45 cm
180 cm man's stats:
TAL = 180 / 2.3 ~ 78.26 cm
AAL = 180 x 0.33 ~ 59.4 cm
If we compare 165 cm man's TAL to 180 cm man's TAL we get 71.74 / 78.26 ~ 0.9166 and if we compare their AAL we get 54.45 / 59.4 ~ 0.9166.
As you can see, the ratios are the same but we have to keep in mind that TAL is more variable between human beings as it takes into account hand and finger lengths which might differ wildly even for the same height human beings (up to 3 cm).
Now, AAL is more accurate and less volatile as it's much more likely that normally proportioned 165 cm man to have 54.45 cm AAL and 180 cm man to have 59.4 cm AAL.
Basically, AAL is a more accurate measurement and TAL is kinda useless as hand and finger length is irrelevant because we're concerned about actual arm length difference between human beings.
Now, 165 cm man's AAL ratio to 180 cm man's AAL is approximately 0.9166. 180 cm man will have 4.95 cm longer arm bones and it will be noticeable even to an untrained eye. If we take into account TAL, now 180 cm man will have 6.52 cm longer arms at middle finger level.
Again, hand and fingers are irrelevant as actual arm bone length is more important so normally proportioned 180 cm man will have about 4.95 cm longer arm bones in total and the most of length difference lies in humerus bone to be exact.
Now, if I, who has about 72 cm TAL and about 54.5 cm AAL (I measured them many times) wants to become 180 cm through LL, my AAL to 180 cm ratio will be 0.9166 compared to someone naturally 180 cm tall who has the AAL ratio of 1 relative to other normally proportioned 180 cm men.
The less I lengthen, the more I will be closer to the natural ratio of 1 of someone my post LL height.
For example, if I lengthen to 175 cm, my AAL / 175 cm ratio will be: 54.45 / 57.75 (AAL ratio of 175 cm man) ~ 0.9428.
If I lengthen to 173 cm, my AAL / 173 cm ratio will be: 54.45 / 57.09 ~ 0.9537.
I think that AAL ratio of 0.95 is the safe limit and the difference will be less noticeable between me and normally proportioned 173 cm man. AAL difference will be just 2.64 cm so it's not that dramatic as 4.95 cm difference I will have compared to 180 cm man. Of course 2.64 cm is still a visible difference but it's not that bad.
So, keep in mind when lengthening your legs to not exceed those ratios dramatically in order not to appear like Henry Cavill next to someone who was born your post LL height.
Also, never exceed 0.51 leg to body height ratio (that is, the sum of femur + tibia length divided by post LL height). Anything higher than that number means less attractive to opposite sex.
Now, if someone is less than 5'5, for example, 5'2 or something, he can disregard those ratios and lengthen 5-6 inches which is the upper safe limit of double LL. IMO 0.90 is the hard limit of AAL ratio until you become so disproportionate that almost everybody will spot that you have shorter arms relative to total body height.
For me at 165 cm original height, 0.90 AAL ratio will be at 183.33 cm post LL height. It will certainly look bad and anything past that height will make me look like a man on stilts with very short arms.
TL;DR:
Wingspan is useless metric as it takes into account hand and finger lengths which can vary up to 3 cm between the same height persons. Also, it takes into account clavicle and scapula lengths, which can also vary up to 5 cm between the same height persons. What matters is actual arm length (AAL) which is the distance from acromion bone down to radial bone bump.
Normally proportioned people have AAL of 0.33 of their height. Your current AAL will determine how you will look at various post LL heights. For example, if you're normally proportioned at your current height and have AAL of 0.33, then, you have to calculate your target height's AAL using the same 0.33 coefficient and then divide your AAL to AAL of your target height.
For example, if you're 165 cm and have AAL of 0.33 which equals 54.45 cm and want to lengthen 10 cm in total to 175 cm height, multiply 0.33 x 175 to get 57.75 cm AAL of someone who was born that height and then divide your AAL of 54.45 cm to 57.75 to get 0.9428.
Remember, anything lower than 0.95 AAL ratio compared to someone who stands naturally at your post LL height will look obvious if you stand side-by-side with that person.
I've calculated the AAL ratio of that 5'3 bald guy who lengthened 6 inches with Dr. Mahboubian and he has about 48.4 cm AAL which is already very short for his starting height of 5'3. He should have 52.8 cm. That means his AAL is 4.4 cm shorter than someone who stands at 5'3. It makes his wingspan 4.4 x 2 = 8.8 cm shorter than his original height so his wingspan is about 150-152 cm. Dr. Mahboubian is about 175 cm, so that patient's post LL height. This meanas that Dr. Mahboubian's AAL should be 57.75 cm. 57.75 - 48.4 = 9.35 cm AAL difference between the two men standing at the same height. That patient's AAL ratio to Dr. Mahboubian's AAL is 48.4 / 57.75 = 0.8380 which is very low, hence why he looks like a t-rex besides Dr. Mahboubian at the same height.
Now, that guy has an AAL of someone who stands at 146.66 cm tall naturally. His wingspan is about 150 cm, so -4 inch ape index to his original 5'3 height and -10 inch ape index of his post LL height.
If I wanted to look like him besides someone of my post LL height, I'd have to lengthen 30 cm and become 195 cm tall. 195 cm man's AAL should be 64.35 cm (9.9 cm longer than mine) so my AAL / 64.35 = 54.45 / 64.35 = 0.8461, very close to that 5'3 guys AAL ratio to Dr. Mahboubian's AAL at 0.8380.
My take is this:
If you're less than 5'5, for example, 5'2 guy, measure your AAL and lengthen up to about 0.90 AAL ratio of someone who is your post LL target height. And, try to not exceed 0.51 total leg to body ratio (sum of femur + tibia bone lengths divided by post LL height). Again, if you're already long legged but very short in height, you can go higher than that ratio but no more than 0.525, otherwise you'll look like a man on stilts.
If you're just normally short guy who is in 5'5-5'7 range, try not to go lower than 0.95 AAL ratio of someone who is your post LL target height and never go above 0.51 leg to body ratio (sum of femur + tibia bones divided by post LL height). If you value your post LL aesthetic appearance of course. Otherwise disregard this advice and do what you want.