I found this source.

https://www.screencast.com/t/2DyXnqbea

I don't know what the "ern" means after each of these numbers. But it's evident that is considered much more safer to lenghten Tibias way more than Femurs.

I have also heard at least one from a member of a LL clinic that you can lengthen the Tibias way more than Femurs.

Here in this forum I hear the opposite.

Is it because the majority is that unflexible that it's because of that THEY can't lengthen the Tibias more, so they conclude Tibias can't be lengthened much further?