I think you nailed it with 25ppm. I use that same test. I find it a little easier to hold the container about a 1/4" above the card over the white area to see the color more easily.
Also, when toy are in the <10ppm range, I think it's easier to look through the side of the container and compare to the higher ppm colors and then divide by 10 to get the final number...ie: thru the side looks the same color as the 50ppm color, then divide by 10 and your answer is 5ppm. Looking down from the top to compare the 10ppm colors is too hard to see the subtle differences!
Not sure if you could call it a reef tank yet, but yes. Mostly zoanthids and euphyllia. Not much algae, but plenty of Cyano (not technically algae). Believe it or not, my nitrates are on a downward trend. Tank is around 6 months, so still maturing.
yeah you are fine I would get the levels closer to 5 and the corals will be super happy but don't go to 0 bad things happen. regular water changes rely help and a refugium with some chato. mostly regular water changes. sounds like your tank is doing though though