Im on CD 2 now, ive got four day periods and a 28 day cycle. I bought a opk kit, but it only comes with seven tests. Ive never used OPKs before so im not sure when i should start using them? Should i just go based on what the apps say is my fertile window? Also is there rules about having to hold urine for them like you would with a pregnancy test? I already know that the line has to be as dark as or darker than the control line so no confusion there, i just dont know when to start testing.