Drilled hole distance question

[pirate]

The new question I have now is when you drill the tank do you take all the mesurments from the edge of the hole or the center of the drilled hole? The math I'm gonna use is
x=1.5T
y=4T
T=glass thickness

holes next to each other next to edge. X from edge 4T apart Y from edge.
And 1 more quick question. Is it better to have the overflow in the corner or in the middle of the tank or somewhere else? Is it a matter of preference?
I was gonna drill the tank today so any help in this matters is welcomed
thanks