aero garden ph down

I have a 20 spot aero table and the res consistantly has 5 1/2 gallons of water in it ( give or take ) I did a PH test and im running a little high at the moment. I purchased PH down from the local dro store and there is not really directions... simply says use spareingly any advice would be greatly appreciated.

I can tell you this..... I use 3 MLs (or CCs) of PH down to drop my PH from 6.5 to 5.5-5.7 for a 2 gallon mix. I went through the same thing when I started to use PH pen. I had to do trial and error to get it right.