Bottom margin vs. descenders question

[Peter Flynn]

On 07/07/2020 12:59, Axel E. Retif wrote:
> The *actual* text box (in movable type, Linotype or TeX) is bigger;
> for example, one can not say we want a text box of 42 picas (504
> points) in height and a baselineskip of 13 points (that would mean
> 38.769... lines): either you have a box of 494 points (41.167 picas)
> to get 38 lines, or 507 points (42.25 picas) for 39 lines; the latter
> will give you about 42 picas of *visual* height to the baseline.

I wish all designers were sufficiently good at simple mathematics to be 
able to compute this accurately. I cannot count the number of times I 
have been given a formal specification by a publisher that literally 
does not add up.

Peter