CB advanced digital OPK

First month using the CB advanced digital opks. I've been using FMU because that's what's on the directions. Should I? Did you? When did you get a + I've been testing for 4 days now but I'm not seeing any progression. Cycles are 31 days so I'm thinking I should ovulate around cd 16 idk... Last month it was about cd 20 idk 😔 hoping I can get a smilie on this in the next couple days! Fingers crossed