I was installing trim on my windows. I tried measuring with a measuring tape, but it was not working out right.
So I gave up on measuring completely. I cut a piece of trim long, held it up to where it needed to be, and then marked with a pencil where I needed to cut. Then I cut it. If it was still a bit long, I shaved off a little more of the board.
Sometimes things are uneven and not square and getting the right measurements never works out right. So I don’t even try.