There is another way to measure for a cinch too. You sit your saddle on your horse, run a string through the rigging on each side and under their belly where your cinch would go. Pull it snug and mark it at the D-rings on both sides. Measure the string and then subtract 16 inches (round off to the nearest increment of 2 inches; 26, 28, 30, etc).
Whether I would want to round up or round down would greatly depend on what type of cinch I was getting.
If I was getting a string girth; alpaca, mohair, even rayon, I would want to round down to the closest smaller size because those cinches will stretch a bit.
If I were buying a fleece on nylon backing, a neoprene, or a leather, I would want to round up to the closest larger size to give a bit of extra room if they put on some weight.