Help with figuring out my dates

Ho

I was pregnant up until May 31 then I had a natural miscarriage they did blood work and confirmed that I was back at zero and tests were negative.

Haven’t had my period since the miscarry. And I’m pregnant again. Took a test Monday and I’m pregnant. Getting blood work today.

But what do I go off of for my last day of period? My miscarry? If so how accurate is that?