I would say start using them about day 10 of your cycle and usually around day 14 you'll get some lines etc. At least thats how it works for me, we're all different though!
So for you, I would start around 30th May
Ps. Afternoon pee is best between 2-6pm.
LOL my OH is constantly being bombarded by the scientific facts by myself. He even started asking if I had done my OPK and what the result was.
xx