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Would be great to be able to see the values as numbers, instead of just sliders, and also to be able to enter exact values.

Thanks.

Take a look at the subject discussion entitled "Parametric Modelling"

I am looking for the same thing but it is very complex. Eg. if you want to lengthen a leg you automatically adjust stature, which then changes leg length etc.

It seems to require a complicated iterative procedure.

I have yet to achieve what I want but I suspect you will need a non-linear optimisation scheme to, for example, set a target height with a known lower leg length and known torso length both held constant and everything else adjusted to "average".