What are the advantages/disadvantages of round and/or oval base-rings?
Some men claim a round ring works better for them than an oval ring, while others claim the opposite. Personally, I don't notice any difference in comfort between a round or oval, and consider ring shape to be nothing but a gimmick.
An oval ring begins as a round ring in the size ordered. Then the ring is squeezed in a vise to turn it into an oval. The top-to-bottom (vertical) measurement decreases and the side-to-side (horizontal) measurement increases.
The first oval rings I made changed shape by 1/8 inch. So many rings were returned due to poor fit that I decreased the change in shape to 1/16 inch.
Now the oval is so slight that the eye doesn't always perceive the shape, and I'm consistently receiving emails from customer's who've ordered an oval ring but think they've received a round ring, when they've indeed received an oval ring.
It's a catch-22. If I don't offer an oval ring I get lots of emails asking about them. If I do offer an oval ring I receive lots of emails asking about them.
You already have lots of other sizing options to consider when building your chastity device. Since the test ring you used to determine your base-ring width is round, then I suggest choosing a round ring for your device.
How do I measure for gap?
There is no way to measure for gap but the 3/8" gap works best for the majority of men. The gap may be easily changed at a later time if necessary. There's no charge for adjusting the gap, other than paying for return shipping of your ring. If you happen to have a bench vise and are handy, I can email you instructions showing you how to adjust the gap by yourself.