Optimal Base Ring Dimensions

Hello, I’m fairly new to this area and have a few questions regarding chastity. I used a string to measure my diameter while flaccid, getting a reading of roughly 60-65mm (the holytrainer website recommends a 55mm base ring for that measurement). My penis alone measures about ~43-45mm in diameter when fully erect. I own an affordable Chinese version of the CB6000 with various rings (sizes: 49mm, 46mm, 43mm, 38mm, 35mm) and also a stainless steel device from China with a 45mm ring. I’ve discovered that 40-45mm rings work best for me when flaccid; they don’t restrict blood flow or cause pain. However, my issue arises during an erection: all the rings except the 49mm one become too tight, cutting off circulation (in the plastic device) or pulling on and darkening my scrotum as the penis pushes forward. Conversely, if I use the 49mm ring, it feels too loose when flaccid… causing it to slide down so it doesn’t sit properly at the base of the penis. I prefer not to have that slipping issue because I could easily slip my penis out of the cage while soft. So, what size would you recommend? Also, is it normal for scrotal color to darken during an erection, and how tight should a ring fit in both states? I plan to wear mine 24/7. Thank you beforehand.:)