Hmm... if someone got 40cm tibia and 42cm femur.
Lengthening the femur by 8cm will make it 40/50, which would be a 0.8 ratio right?
Would it be worth to do so if your inseam represented 50% of your total height in that case?
Yes, that would make it 0.8.
Depends. How long is your wingspan?, that's another variable you have to also consider when deciding how much you should lengthen.
My inseam is 83cm and my height is 172.5cm. If I were to lengthen my femur by 6cm, it would make my inseam be 49.85% of my total height. If I were to lengthen both my femur and tibia by 5cm, I would then have an inseam which would represent 50.95% of my total height.
Below are LL mockups which show how I would look like with such inseams. With an inseam of 49.85% or lengthening my femur by 6cm doesn't look bad, well at least that's what I think.
https://m.imgur.com/g9zFpSB