When measuring material to be cut I find it helpful to put a little tick mark of the side to cut on. This simple action saves countless time remeasuring and helps me account for kerf , which is the thickness of the blade you are cutting with.
Kerf is the divergence between the left and right sides of the saw teeth, and any cutting will result in some loss of wood that is turned into sawdust. If you just cut directly on the marked line the blade kerf would eat into your measured piece, causing your cut to be inaccurate. This may not seem like a big deal, but if you've ever had a project be a 1/8" out of measurement you know how frustrating this can be. The solution is to measure your piece and make a small tick to one side of the measured line, indicated which side to cut on. Once measured, line your blade up to the line with the blade on the ticked side, so that the kerf will be on one side of the line and not into your measured area.