I did not want my hole too close to the edges of the glass. The close to the edge you drill - the more likely to fail and crack it. Drilling in the MIDDLE is the safest place, but who wants to do that? Not me.
I had to drill a 60mm hole for my 1.5" bulk head. Thats a 2 and 3/8" hole. The OD is much larger than the clear ID when it's all piped.
I made sure the outside edge of my hole was at least 60mm from the top of the tank. One full hole saw width is as close as I wanted to get. I went about that same distance from the outside corner too. 60mm down and 60mm from the left edge. Zip! Zip! :mrgreen:
So the hole is 60mm, but how big is the flange on the bulkhead? You want at least 1/2" clearance between the edge of your inside flange and the box. Have you measured your bulk head yet? Outside flange dimension? The box has got to be at least that big + about 1/2" below the flange on the very bottom.
How did I come up with that? Well, ya see, I'm a redneck. I like to use a tried and true engineering principal from way back. It's called "That looks about right!" :mrgreen: