Here's one way you can get a "ball park" figure for the gap measurement. (see what I did there
Get two pencils and two elastic bands.
Place one pencil on the front of your scrotum above your balls, and the other pencil behind your scrotum level with the front one. Loop an elastic band (gently) around each end of the pencils to hold them there. Pull your balls down so that the minimum of tissue is trapped between the pencils.
Now you can gently arrange the pencils so that they're parallel, and it's then easy to measure the distance between them. The measurement that you get is the minimum distance that the largest part of the cage gap should be.
You could also check the largest gap that will keep the cage on (but this isn't quite so accurate).
Take the elastic bands off and hold the ends of the pencils in each hand in the same position. Now gently pull the pencils off your balls, allowing them (the pencils) to separate until the smaller ball pops through. A gap larger than the distance between the pencils will be ineffective in holding the device on, and to allow for humidity/temperature/arousal/... changes, should be a few millimetres smaller.
Repeat both measurements and work out your minimum/average/maximum values for each measurement. The difference between the minimum and maximum measurements will give you an idea of just how critical your own gap setting is.