so if i do a side hole i should do it as far down as possible??
No. If you use an OF box with a side hole, it will be near the top. You would almost need to build your OFbox first and use that for getting your measurements. You would also want the hole higher up for there is less pressure at the top of a side than at the bottom of a side.
Will you be drilling the tank yourself? If you are... I would suggest against it. I have tried drilling glass, tempered and regular, and it is really not fun.
Tempered is a whole different animal and I would highly suggest having a professional do it, if they would even do it.
If you do drill yourself... take your time and if you are succesful, use a dremel and sand all the edges down to remove the micro fractures that occur from drilling.
I would suggest building your OF box out of acrylic as it is much easier to work with and like someone mentioned earlier it won't break if rockwork falls onto it for whatever reason.
Someone also mentioned about when power goes out and extra water flowing down into sump.... all drilled tanks will do that... that's why you fill the upper tank and let extra flow to the bottom till the sump is filled as far as you want it at maximum... then when the pump turns on the water level in the sump goes down... mark a line on sump for the water level when pump is on and when off... makes it much easier to do water topoff while system is running.
To keep things more simple... I would suggest a WEIR overflow. I have had mine running for quite a while now and even had the power going out the other day and it restarted no problem *thumbsups
I also simulated power outtages over n over and it restarted everytime. The downfall is that it does build up some airbubbles but about once every 2-3 weeks I suck the air out, NBD