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Tx, see that's something concrete :)
As I asked what an objective measure is of functional flexibility in the context of cycling (as I am clearly too stiff to do ballet) this may work.
For example: You put feet against wall, then if you can't touch something that sticks out that x% of your leg length in your picture position you need to stretch, a lot, and now.
(though that makes it dependent on relative arm to leg length again...sigh...)
One possible way