I am not sure we have answered well the OPs post.
I guess this would be somewhat of a math challenge.
If i interpret the OP correctly, what he wants is a calculator that would predict the effects newarts displays in his graphic.
in math terms he wants
shooting distance = Function(focal length, limit distortion0
I don't know the function needed, but suspect it is related to lens magnification, or the way to express distortion, but expect it would be the difference in magnification for the focusing distance +/- about 100mm (thickness of a head?)
Unfortunately as someone pointed out, this would need different distortion limits for someone sitting, to avoid exageration of the feet, etc.... but you see where this is going.
Not sure anyone has developed this type of thing other than the graphics such as what newards presented.
It is really something learned with experience not defined mathematically, althouogh it would appear possible if you had enought time.
Me, I would rather go out and tape pictures