Welcome, Carrie! The FSH question is complicated. Yes, it should be taken on a certain day if you're still cycling. But, in perimenopause, even on that particular day, it could really read at any level, as hormones fluctuate up and down in peri.
As for your cycles, that is very similar to what I have experienced as well. I'm 39, I had clotty, heavy periods, and extended period of bleeding. They did ultrasounds and biopsies to look for other causes. Then when I had my second round of skipping periods for 3 months, they tested my fsh and it came back well into post-menopausal range.
My guess is, with your symptoms, if your fsh come back high, you can probably trust that it's menopause. But if it doesn't come back high, that doesn't mean you're not in perimenopause, it may just mean you're blood test caught you in a fluctuation of higher estrogen and lower fsh.