Gotcha! I thought it was like one of those wire gauge cardsNo, that's not how it works.
You can just walk up to a window, hold the card against the glass, tilt it, and read the thickness. You don't need more. You don't need to see the edge. It works on the fact that glass bends light and thick glass bends it more.
The tank must be empty though.
I actually quit reading after the first sentence and I never got to this part.. . . I plan to use bulkheads and I want to be sure the glass isn't too thick for the threads.