A canonical US manuscript page contains 250 words: 60 characters (1 word = 5 characters + 1 space, * 10) by 25 double-spaced lines.

That is, of course, on a typewriter.

Setting OpenOffice Writer to a US Letter-sized page with one inch margins and a Courier New 13pt font yields the 60 characters; to get the 25 lines exactly, I set the default paragraph style’s Line Spacing value to Proportional — not Double! — and the percentage value to 178%.