Hey,
If you check out the human measurements data:
http://www.roymech.co.uk/Useful_Tables/Human/Human_sizes.htmlhttps://msis.jsc.nasa.gov/sections/section03.htmYou'll see that for the most part height is in the legs (tibia, femur) as sitting height is similar. In general, the recommendation is to not go past your wingspan (+/- 2.5cm or so) for leg lengthening so that proportions look ok. The caveat I can think of are if you have really broad shoulders (clavicles) and short arms so even if wingspans being equal, you may feel arms look short.
The other thing is, tibia is for the most part about 80% of the length of femur (preserved over different ethnicities, probably some mechanical reason to it). So, you want to lengthen femur, tibia about equally if you want to lengthen alot. The general consensus seems to be up to 5cm per segment gives you acceptable proportion and post surgey function. It is accepted that in doing >5cm in one segment vs. 2 segments 5cm each, the 2 segments results in better function (less stretch of muscles, nerves).
Ie. in summary, you can lengthen:
1) (wingspan - current height) +/- 2.5cm if around 5cm either tibia or femur,
2) 5cm on each tibia and femur for a total of 10cm.
This is given you mind proportions, some may not care and there have been patients who've done as much as 9-10cm on tibias but this is not recommendation not just b/c of proportions but b/c you won't have good function afterwards. The tibia proportion is noticeable when you sit down.
Here is a guy who went from 170cm to 178cm doing 8cm on femurs.
If you have the funds and you want the 7-8cm like you said, I'd say think about doing it in two segments and do it with an experienced doctor. You really want an experienced doctor and not go cheap on this b/c even though you will have better function and proportion with two segments, you want to avoid getting knocked knees (each tibia and femur lengthening causes knock knees on their own) so that you don't get arthritis in the future.