It's hard to say on the Phosban without knowing how much phosphate you're adding to your tank via food, salt mix, water, etc. Most people say every 6 to 8 weeks, but that's only a rule of thumb. You could need it more or less frequently. Some people have a good idea of what their tank looks like and change it when their corals start looking unhappy. Some people change it when they see the beginnings of an algal bloom. The only real way to know for sure is to get a super accurate low range phosphate kit, which can be cost prohibitive. I'd start off with about an 8 week schedule and move that up if I notice algae coming back. Also, I'd skip the Phosban and get bulk GFO from someone like BRS. Phosban works, it's just expensive for a phosphate remover.
As to the testing pH after the lights go out, it's not wrong, but it gives you an incomplete picture of what's going on in your tank. You would want to test after the lights go out to see how much your pH lowers at night, but you also want to test during the day to see what the daytime pH is as well.
There's a natural swing in pH that goes on in all aquariums during the light and dark cycles. When the lights are on, corals and algae use light and CO2 to photosynthesize. A byproduct of this process is oxygen. Using CO2 and releasing O increases the pH. At night, this photosynthesis doesn't occur, so the pH is generally lower. I'd definitely recommend measuring after the lights go out, but I'd also measure during the day as well.
