Mature Metal discussion

I’ve made up my mind to dive into the world of chastity and have settled on ordering either a Mature Metal “Queens Keep” or a “Jail Bird”. I’m currently in the measuring stage before I place the order. My measurements show I fit inside a tube that is 1 ¾ inches long with a 1 ½ inch diameter. I’ve been wearing this and noting that usually the tip of my penis sits right near the end of the tube, though of course it extends further when I’m aroused (I’m a grower) and can sometimes retract about 1/8 inch inside. I’m measuring from the tip to the balls, at the base of the penis, while soft. I’m not entirely sure if this is my true resting state or if it’s a bit shriveled; I’m still a bit confused by it. I feel that if the length were exactly 1 ½ inches, I’d have zero room to breathe and be completely constricted and tight, which I don’t think is the goal. I’ve seen advice on this forum that for Mature Metal devices, I should subtract a quarter inch from the length measurement due to the design. If that’s accurate, a 1 ½ inch length would be a perfect fit, but I definitely don’t want to go too small. I’d prefer to be ¼ inch on the long side rather than too short. Regarding the base ring size, while trying on cock rings to get an idea, a 2-inch ring is too loose (I can fit a finger under it), but 1 ¾ inches is snug. Therefore, I think I should probably choose a 1 7/8 inch base ring and hope for the best! I can wear a 1 ¾ inch cock ring all day without issue, but it is tight, and with the cock cage component of the Jail Bird, it might be slightly uncomfortable for long-term wear, which is exactly what I’m looking for. I really appreciate the wealth of information available on this site. I’d love to hear your thoughts on measuring for a Mature Metal device. I intend to place my order as soon as I’m confident in my final measurements, having read through all the posts here for guidance. Your feedback is sure to be helpful! Thanks