Gary,
I've found some interesting ways to use the Ideal-scope very effectively. Some background: I recently purchased a 3 stone setting with an eight star center and regulars on the sides. The firescope images are emblazened on my memory and I can replicate those images with the Ideal-scope, even when the stones are already set.
With the set ring completely out of focus and slowly bringing it into focus, but still hazy, you can see where the light leakage exists for the non 8*'s, just like the firescope. The 8* is the same in all conditions, in focus, out of focus etc. I think what's especially required with set diamonds is that you have light enter the crown while the diamond is outside of the cup of the ideal-scope. Basically have the light source perpendicular with the crown. Using this methodology, I can replicate the images I viewed on the firescope before the diamonds were set.
Being out of focus shows me light leakage, where as in focus demonstrates the amount of black areas.
Your thoughts?