Your mainline will neck down as it gets moves towards the end of the line and drops. Prob to 10" then 8". It does not stay 12" all 90'.
I use a a 10hp cyclone with 12" inlet and Nordfab ductwork. Shed is 40X64 and have a mainline up 6 to in rafters run the full 64. I drop to 10" at like ~25' when I Y drop to my jointer planer cluster, take 10" have a 6" off shoot to router station and 10" to middle and have a y to the north that is 10" that goes 25 and then necks to 8' and drops two different 8" drops fopr sanders and bandsaws, and back at mailine 10" continuing west to like 50' getting a 8" drup to tablesaw/and overhead collection then I neck off mainline to 8" at 50' that drops to a shaper.
Point is you dont want 90' of 12". Id do some math on your needs based on tools and the run characteristics etc. read up on air handlings ductwork sizing guide.
Many people who run PVC run complete 8" or 6" mainlines cause they do not have pvc options like metal ductwork will offer you. You want the best setup to support your layout and what is best when moving chips from machines and keeping them in flight.
Glad its my shop I am responsible for - I only have to make me happy.