The general rule on drilling holes in glass is that the edge of the hole should be further away from the edge than the thickness of the glass. This means that the edge of the hole on a 6mm thick piece of glass must be more than 6mm from the edge of the glass.
The calculations are simple arithmetic. You calculate the centre point of the hole by adding the radius of the hole to the thickness of the glass plus at least 1mm. For example, to drill a 10mm hole in 6mm glass, you add 5mm (radius of hole) to 6mm (thickness of the glass) plus 1mm = 12mm as the minimum distance from the edge of the glass to the centre of the hole.
Remember this is the minimum distance. For safety and durability in architectural or heavy circumstances, an additional margin must be added.
For other tips on glass drilling see:
Keeping things wet
Using a drill press
Drilling with a Flushing Head
Drilling holes with copper tube and grit
Drilling glass without a drill press
Drilling speeds for diamond bits in glass