OPK Question

I'm going to use an opk for the first time but I am unsure what CD I should start testing?! I use three different apps and they all show different "green weeks" although they're all close to each other. If I generally have a 29 day cycle (little off this last month but is usually spot on) what day should I start taking the tests?! Sorry for the clueless question...everything I read gives me different info so I want to ask you ladies 😘