When the year ends with one of the letters of מנצפך - for example 5780 - What is the correct way to write it תש"פ or תש"ף?
The Lubavitcher Rebbe writes that while he did not receive an explicit instruction, the custom among many old printers (many of whom were big Torah scholars) to write the simple and not the final letter(look in most gemaras on Daf Chof, for example). Practically, most letters that were written in the Chofs (the 60's) were written with a smiple chof.
I've seen others signing with the final letters (see some of the letters of approbation to Artscroll's gemarah which were written in the Nuns).
Incidentally, in gematria, the final letters have their own gematria: chof is 500, mem is 600, nun is 700, pei is 800, and tzaddik is 900.