I never said that there was any magic formula. I am just saying the the "under 6cm tibia and you will be safe" will most likely not apply if you are 150 and have really short tibias. So when setting a goal for yourself. Remember, I am talking about goals. Doing it % wise is a better way.
And also, since most people don't get the big issues before 5cm which is under 15% for most people. Setting a goal of 15% on each bone would be wise for me.
For an example @YourSpaceBoyfriend, your goal is 6cm femur and 4cm tibia. Doing the math most likely it would be wiser to do less femur and more tibia. Since your femur is not 50% longer than your tibia. (Not attacking you just taking your goal as an example since I think it's a very good and realistic one.)
This is just estimates as I find the math fun, and people with an iq over 105 will understand that.