To be easy and help witu juggling maintensnce and construction, you could say that ships stationed at a home planet at begin and end date of a pay cycle/or for a min amount of days in it, get a maintenance cut? I.e. 25-50%. Then its just basically maintenance cut for pdf ships.
Or just that once you declare a ship on pdf, if its been there at least 20 days come next pay cycle, maintenance cut on it. You kinda create a "war season" there tho, pkayers would seek to launch offensives in first 10days of month, get some ships back home in time to save the money and harvest the crops. Makes for a little deployment kinda scheme
Orrr just plain you have to assign the ship to pdf and leave it whole cycle to get cut. If move at all, or not there from 1st to 1st, ts normal maintenance.
Just thoughts