Assisting in Choosing the Right Base Ring Dimension for the Jail Bird Apparatus

Hi there, I’m thinking about buying a Jail Bird chastity cage for my slave husband, and we’ve got the sizing rings from Mature Metal. We’ve had him wear some of those red plastic ones for several hours at a time, which helps me narrow it down, but I’m still not sure exactly how to pick the right size. How snug or loose should that base ring be? If it stays on without sliding off, is that okay once the cage is attached, or does it need to fit tighter? My main worry is cutting off his blood flow. Best, Alex