Determining the Right Cage Length for a Jailbird

Hello everyone. I’m new to posting here but have been reading along for a good while. The community seems very welcoming and friendly. I’ve tried out various inexpensive plastic and metal cages, and now I feel ready to buy my first genuine Jailbird from MM. But before I do, I have some questions about what I think will work best. For the ring, I’m going with a 1 7/8” size. The gap should be 3/8” wide. For the tube diameter, I’ve chosen 1 5/8 inches (it’s slightly bigger than my exact measurement of 1.6”, but if the cage is shorter, that extra width shouldn’t hurt). The main issue is figuring out the right cage length. After measuring myself numerous times over several days, I found that the distance from the tip of my penis to where it meets the scrotum (along the underside) is exactly 4 inches. This matches how MM suggests you measure. I also own a curved plastic model and two metal cages—one resembling a CB6000 style and the other more like a Jailbird. Since these are curved, I assume we should measure along their centerline from tip to ring end. If that’s correct, my curve cage is roughly 4 inches long, while the two metal ones measure about 3” and 2.5” at the centerline. The 2.5” fits me best but feels just a bit too tight/short. With the 3”, there’s often some gap open, though part of that might be because the tube narrows toward the end, causing my penis to compress rather than fill out completely. Questions:

1. When MM asks for cage length, do they automatically subtract the gap amount, or is it a direct measurement taken at the centerline?
2. Based on these numbers, I’m leaning towards ordering a 2.75” cage because I want something slightly longer than my smallest metal option. Does this make sense given a 3/8” gap and a flaccid (underside) length of 4 inches?