Most of the dimensions I've ironed out:
Total length: 53mm
Penis ring: 35mm diameter
Cock ring: 45mm diameter
Urethral plug: 8mm thick, no ball, extending 10mm beyond cock ring.
Gap: UNDECIDED
Determining the width of that gap is the tricky part. My testicles ride high and tight - if that gap is too wide, out they pop. Too narrow - pain. The bad kind.
My Jailbird had a gap of 3/8". That's about 9.5mm. Never gave me any problems. Because the base ring is oval, the two rings were about level at the bottom, so there was no added length due to differences in height.
That won't be the case on the Looker. I've made a little illustration to show my reasoning and indicate my concerns:

The metal making up the penis ring is 6mm thick, that of the cock ring is 8mm thick.
If the two rings are level with each other as indicated, then Line B = 11mm. All the reasoning below assumes this is the case. If anyone can confirm / deny this detail for me, please let me know.
What I'm concerned about is Line C, as that is the real gap keeping my testicles in place (actually you have to subract 7 from that value to account for the radii of the two rings).
So,
If the horizontal gap = 10mm, Line A = 17mm. Line C = 20.25, then subtract 7 for the ring radii and True Gap = 13.25mm
If the horizontal gap = 9mm, Line A = 16mm. Line C = 19.42, then subtract 7 for the ring radii and True Gap = 12.42mm
If the horizontal gap = 8mm, Line A = 15mm. Line C = 18.60, then subtract 7 for the ring radii and True Gap = 11.60mm
So, questions: Have I done this right? Did I miss anything? Looking at these numbers, I am very tempted to ask Dietmar for an 8mm gap. When I used to wear a CB6000S, I used the 1.75" base ring with the 1/4" spacer without discomfort so I think it'll be okay, but I would very much appreciate any advice on the matter.
Thanks.