Excited to see how your first OCLT goes! As long as you're using Taylor chemistry for the ph test, any FC under 10 is okay to do the ph test on.
Did you do the FC test after your vac? Did you bring it back up to 12 or doing OCLT from 8.5?
I'm not an expert on CYA levels for your area, but as long as you're only losing 1-3 ppm/day you're good I imagine. If you're losing 4 for several days in a row, I'd go 40 and see if it helped. Let's not add CYA until after you pass an OCLT or two.