Drilling a tank!!

[muck]

Im going to drill my tank for an overflow... and I have a few questions.
I am thinking of drilling 3 holes in the middle back bottom of the tank.

1. What is the minimum distance the holes should be spaced apart??

2. What size holes should I drill for the overflow?? There is to be a main line to sump and one backup.
(Just in case... )

3. What kind of turnover ratio should I be trying to achieve for my 40Gal Tank??
Will probably go with a Mag Sumbmersible pump in the sump for now.
Would a Mag 9 be enough flow??