Where should I measure before shaping begins?

I need to knit 10 inches before shaping begins. Do I measure the longest area (far right)? I’m assuming the piece will even out with blocking?

BTW, this is Walk by the Seaside from Knitpicks book “Take It Easy knits for relaxing,” and what you are seeing is the top back of the cardigan with provisional cast on where hood will be added - Holly

Welcome to KH!
What pattern are you making? If it’s mostly stockinette, the longer sides, I’d measure there.

Thanks for the advice =) It is a hooded cardigan that has that garter stitch pattern in the center back, and I’m pretty sure that is what is pulling up the center

Yes, it is. The same number of garter stitch rows will be shorter than the same number of stockinette rows. You can adjust for this by doing a short row periodically in the garter stitch section. That way, you’ll add extra rows with the idea of equaling the length of the stockinette section. Blocking will hopefully help too.