Spiral (Twister 2) ring size

I recently bought the Spiral CD from Tickleberry which is available with 3 different ring sizes, 40, 45 and 50mm.

I bought mine with the 50mm ring, when I've completely shrunk, I can pull my penis out as the current ring size + the fact that my balls hang low means I get a fair amount of space between my body and the ring.

I think if I get a smaller tighter ring, it will stop the ring being able to slide away from my body and therefore stop me being able to remove my penis from the cage.

When I'm completely flacid, my measurement should be 45mm whereas 50mm is more suitable once I grow a bit but then it's pretty pointless having the CD seeing as I can just remove myself from it anyway when flacid.

So my question is will 45mm be a noticable difference and give me that level of security to stop me from slipping out or would I have to go down to 40mm to get a secure enough fit?

I don't want to end up getting blue balls but it's pretty useless with this ring size.