How far along am I?

I’m pregnant but not sure how far along I am. The first day of my last period was July 2nd (my cycles are 28/29 days) but I didn’t get a faint positive till a week after my period was late (all negative before) does that mean I’m not as far along as I think cause my positive didn’t come till a week later? Will I be measuring 6 weeks Instead of 7? Really don’t know how this works... trying to keep track of things.