Introductions discussion

Hi all, I’m brand new to the world of chastity. My girlfriend and I find it really intriguing and thrilling. I recently bought a CB6000 but have one question about fitting: I’ve been using the largest ring size and it’s quite comfortable, yet I can still pull my cock out of the cage. Then I tried the third size down; it’s tight enough that nothing pulls through, though I’m worried it might be too snug and restrict blood flow slightly there. There is a slight color change but no real swelling, and the veins in my testicles are more prominent/sticking out. Is this the right level of tightness for chastity, or should I switch sizes to allow some pull-out? It makes me feel guilty knowing my GF won’t have 100% control if I can slip a bit free. Any help would be greatly appreciated…