Mature Metal discussion

I feel pretty confident about picking up one of those MM devices that everyone’s discussing. I plan to grab some sizing rings and take a ton of measurements to ensure I get it right. However, I’d prefer a cage with shorter length. Not excessively small. I understand the advice is to subtract just a bit from your measurement when determining length, but what if I deducted more? For instance, if my measurement was 2.5 inches, would choosing a 1.7-inch cage work?