What colour is your drop checker ?
You could use multiple drop checkers with indicator/solution diluted 50:50 with distilled water to read colour change at 30ppm, 15ppm and 7.5ppm ?
Haven't yet installed one.. And i have to search for it, i have only one left somewhere. Not have used any co2 for over 2 years.. Than i have to make me some fluid too.
Seeing the ph profile from previous experience it think it should be on the tipping point of green. I'll cook me up some dkh4 and see what it tells us.