I have an article that should answer all your Q’s here!
It can be confusing I know, but just remember that the basic gist is to do the injections at least 3 weeks. Anything beyond that up to the 6 weeks there are no rules – so you can stop hCG on day 27 or day 29 or day 34 – it’s all okay. Once you decide to stop injecting, 72 hours from the moment of your last injection, is when your first moment of P3 starts- the link above has a handy chart with days of the week to show you so you don’t have to do the math.