Novices and newbies discussion

Hi there! I’m writing my second message here, and I am currently in the middle of purchasing a custom cage. The title asks whether it is better to subtract either 1/4” or 1/2” from your actual measurement when determining the length for a new cage, and also why we do this subtraction at all? Many sources recommend taking off 1/2”, but the maker I am considering suggests deducting only 1/4”. My current measurement is 1.57”. If I subtract 1/4”, that gives me 1.32”; if I go with a 1/2” deduction, it drops to 1.07”. The manufacturer in question provides cages in lengths like 1.25 and 1.5 inches. This seems to point toward the 1.25” size being the ideal choice, correct? However, I would appreciate understanding why we deduct from this measurement so I can grasp it more fully? I am also curious if there are any significant issues with a cage that is slightly too short for long-term 24/7 chastity (meaning just a few millimeters shorter than your flaccid length)? Thanks ahead of time for your answers!