Hole location for Herbie for overflow's

[Palmer]

Hello,

I am planning on drilling a couple of hole's in a tank to make an internal herbie overflow. The hole's are going to be 42mm (I believe dont have the saw size with me at work) to support two 1 inch pvc pipes for a frag tank.

Queston- Is there a standard minimum distance from the edge of the tank you shoot for to maintain structural integrity of the glass? Or is something like 1 inch enough? I am putting these in a standard 30 G all glass tank.

thanks