6.5 cm femurs
4.5 cm tibias
I think you can retain up to over 95% athletic ability. You either fully recover or you don't. There is not much in between.
There’s a lot in between. No one fully recovers athletic ability and very very few never recover at all. I’d say something like 99.9% are somewhere in between.
As for retaining 80% of your athletic ability. No one can say for sure as there’s no objective way of quantifying it.
But the general rule is that less lengthening is better for athletic recovery. Something like 4 cm femur and 3.2 cm tibia (retaining the 0.8 tibia to femur ratio) should be optimal for athletic recovery, but still you depend on hard work on PT/lengthening and everything going well that is outside your control.
Maybe something like 5 cm femur and 4 cm tibia (retaining the 0.8 ratio) and spreading it over multiple years.
One argument is that it is better to just do 5 cm femur as you are constraining it to one surgery, as each surgery has a cost on athletic ability, so one is better than two.
Another argument is that retaining the 0.8 ratio is better in the long term, so going for dual surgery is a bit of a gamble but could pay off long term by retaining the ratio.
The only thing we know for sure is that if you go for only one surgery then femurs are better than tibias for athletic recovery. And the second thing we know for sure is that less lengthening increases the probability of athletic recovery (although you’ll never recover it 100% you can still recover a lot, but it takes time).
But again, we don’t have any studies when it comes to recovering athletic ability, so take my post with a grain of salt, these are just my thoughts.