Calculating due dates

What is the best way to calculate your due date and how far along you are? Last period or date of positive ovulation test?

My LMP was 9 September so using that I have a predicted due date of 16 June 2017. Making me 6w 4 days.

I got positive ovulation tests on the 23 and 24 September so based on that my due date is about 1st or 2nd July 2017. Making me 4w 4days

Have I done this correctly? There is a huge difference in how far along I am and predicted due dates. I believe the Dr will use LMP? But will they adjust their estimates based on how the baby is growing?

Sorry for the load of questions but my doctor's appointment is so far in the future and I want to know this now cause I'm impatient. 😁