It isn't an "assumption", it's a fact
Nothing personal, but the concept
of "lines per page" is as contrived & archaic as "2 spaces between
sentences" and using empty paragraphs to separate paragraphs containing
text. They all revolve around the limitations & constraints of a fixed
monospaced mechanical device called a typewriter.
Contemporary word processing is virtually the same as type-*setting* rather
than type-*writing* & incorporates automatic spacing adjustments predicated
on typesetting standards - proportional spacing rather than fixed. So even
using monospaced fonts the width of characters & spacing between them can
vary even if the font size remains unchanged. IOW, changing from one font to
another can alter the number of characters per line which will alter the
flow of the text, and there's even more fluctuation with proportional fonts
designed to avoid crowding & gapping of the text - enhancing readability.
Before you added the page numbers your document did not use the Footer area
so you were able to "force" the 25 lines. However, once you inserted the
page numbers the Footer claimed its rightful space which reduced the amount
of vertical space on each page available for the text.
It isn't that it's impossible to do what you want to do, but all of your
parameters need to be calculated & set *first* - otherwise you wind up like
the proverbial dog chasing its tail every time you change something or add
something new... Can you imagine trying to play football on a field whose
dimensions are constantly being redefined & the rules keep changing?
Regards |:>)
Bob Jones
[MVP] Office:Mac