Depending on the cock ring diameter And thickness, and what you aim for. Most cock rings nowadays designed to accommodate you 70% of your fully erected girth at the very back bone pressed base. However, having it anywhere else, the same applies. The rings are accommodating to the expansion by allowing flexibility without pinching or over constricting. Me, personally, I like to use the ring that is 60% of my erected girth at the base, giving that extra constriction to add an additional glans flare during hyper sensitivitive moment.
The thickness and wideness of the ring also plays a role in providing extra ring shape rigidity and constriction. The thicker and wider the ring, on top of smaller diameter, the hard the constriction role it plays in holding the blood in the chambers. However, you can't cycle the new blood in fast enough to prevent the penis from turning blue, purple, and numb.
At 80% of erected girth diameter, I lose 50% of my erection without stimulation in 3 minutes. At 70% ring diameter to my base girth, I retain 100% of erection all the way in until I have no use for the erection, and drop of 80% erection if I have no stimulation in 4 minutes. At 60% of the girth diameter, it's 100% erection with numbness after 30 minutes, and drop to 90% erection without stimulation in 5 minutes.
Hope that gives you some side by side comparison to yours.
"Click" the link, to buy at MoS Shop.