The experimentation was worth it, I think we picked the correct measurements, no adjustments required.
At first I was using silicone lube every morning and night, but yesterday I got brave and decided I don't actually need lube, the device fits just right. It fits even better now that it isn't sliding around

Sir says it is never coming off. That turns me on, but I don't actually believe him, he likes playing with my cock occasionally, so I bet he'll order me to take it off every week or two.
Total length: 2 1/4"
Cock ring: 1 1/2"
Spacer: 1/2"
Cock diameter: 1 1/4"