Hello budy, how are you ? I will use my measurements to make it easier to understand.
The final proportion will depend on its current proportion, biologically the femur is the longest bone in the body getting up to 59% higher than the tibia, but in my case I have the femur ratio 50% and tibia 50% (42cm and 42cm ) then a stretch in the femur of 8cm would be femur50cm and tibia 42cm the proportion would still be femur 54% and 46% lukewarm my proportion would still be normal.
Another very important issue is the wingspan, in my I am 1.78m tall and 1.84m wingspan, and most importantly my relaxed arms down are in the middle of the length of my femur,
if I stretch it 8cm it still gets propocional below the waist, for people with arms up to the waist the final proportion with pernar longer can get strange even.
so take your measurements, do the calculations and take photos, edit the photos to get an idea of how the final proportion can be.