Inseam means here: inseam (measured) to the floor - (synonymous to crotch height.)
Germany, year 1999) the statistical inseam value was near 47% of
the body height - (almost same for man and woman).
Thus, for the statistical grown human 20,5 of inseam would be 9,6% of the body height.
Then, a rule of the thumb could be: body-height[in cm] minus 8 = middlecrank (in mm) *)
This applies only to the average proportionated - so referring to the individual inseam makes always more sense.
And always feel free to subtract or add up to10mm as a personal preference (shortcranker/longcranker).
*) BTW - all this cranky cranksience seems - up to now - only refer to males.
(Maybe this will change, after lascivious woman start hunting on bicycles after their preferred males.)