Trying to measure is very difficult - you really are best off starting with a Miller device (CB3k/6k/6ks depending on your flaccid length) and using trial and error. They come with lots of different sized rings and spacers so you are 99% sure of being able to find an appropriate combination. This for a couple of reasons:
1. If you try to measure and get something made the chances are very small that it will fit
2. Even if it does, when you first start wearing a device your anatomy changes - things stretch, and so it will only fit for a short time anyway before needing adjustment. I found it took about three months for my size to settle down.
In terms of how to get the right fit with a CBXk, there are a lot of threads here which discuss it, but in summary:
1. Find a ring that is one size above tight
2. Use the smallest spacer that you can use without pain.
Over time you will find that you start using smaller spacers and potentially then a smaller ring. Important is that it's the gap (spacer size) rather than the ring size that keeps everything tight. Timing wise I found that in the first three weeks I reduced spacer size quickly and thereafter a lot more slowly. I'm still using the size that I got after three months. When you think your size is settled, then is the time to go for a custom device, and the mesurements you need are there for you. Also important for this is that you are kept locked most of the time early on, as otherwise things "unstretch".
Hope this helps