Birdlocked discussion

I’ve been working on finding the right fit for a Birdlocked device and would appreciate some input from those with more experience. I currently use a 40mm metal ring that sits comfortably, but when I get an erection, my penis and testicles don’t circulate blood well enough and start to turn purple, which means I can’t wear it around the clock. I think this size is probably right, especially since using a device limits how fully you can erect anyway? Do you suggest choosing between 40mm or 45mm? Cheers.