Why don't you use a scrap and cut a few different squares (use the 2" so you don't waste a lot). Pay attention to where the line is, cut, and then measure the piece you cut.
I also use the yellow/black rulers and have my eye trained to read the black line on the fabric edge.
Make sure to also sew a trial piece and measure before and after pressing. A lot of times we have correct seams but press some of fabric down instead of open.
It's great that you challenge yourself to do it correctly from the start. Once you get the magic combination, you'll be flying through those projects.