Question about OPKs

So, AF is almost gone, and I am gearing up for the next cycle. Since I keep missing my window with testing, and am trying to confirm/ track this for the dr, I wanted to check with you ladies. 
When do you start using OPKs if you have averaged around 27/28 day cycle for three months?
I have the clear blue advanced digital tests (purple tests) + strips, + clear blue digital (pink). I know what the tests suggest, but is cycle day 6 too soon? (Will testing this close to AF give a false positive using purple test?)