Accurate counting with Flo?

I used the period tracker “Flo” for the last three years & was borderline obsessed with accuracy while tracking. My question is have any other expecting moms used it? Was it accurate with “how far along/due date” for you? Mine says 8w 6d, but it claims to be tracking from the first day of my last period, but when I calculate the same date into my pregnancy app it’s showing a whole week+ less. I haven’t had my first appointment yet & I told myself I was just going to wait, but it’s driving me absolutely mad!