starting my DIY overflow box/sump tomorrow - last minute questions

[jacksonpt]

I'm going to Lowes tomorrow to pick up some PVC, 1/8" acrylic, and silicone to start my DIY overflow box. I wanted get a few things straight before I started...
Overflow questions:
1) what's the purpose of a bulkhead? Why can't I just cut the hole to fit the tubing and then silicone the fit?
2) Does the size of the overflow box matter as long as it's big enough to allow enough water to overflow to keep up with the return pump?
3) I think I want a mag drive 7 for the return pump. It's rated at 700gph, but according to a cometitor's website, it will give approx 420gph after the water is pushed vertically 5' back to the tank. That's a good amount of flow for my 44g reef. I plan on using a ball valve to help regulate the mag drive, but I want to get the syphon flow about the same as the 420gph from the mag drive. How can I estimate tubing size for the syphon that will give me that kind of flow?
Sump questions:
1) I've seen most people's DIY sumps have dividers in them, relatively closely spaced. Is this just to help eliminate micro-bubbles?
2) Water flow rate through a sump doesn't matter much, right? Flow is more important in refugiums than in sumps, right?
Thanks for the help everyone... I'll take some pics and maybe do a little writeup of my project (assuming all things go well).

 

[hondo]

the mag 7 is fine for your tank I use one for my 50. water will only syphon out of the tank at the same rate it is returned so you do not need the ball valve on the syphon side but do put one on the return so you can reduce the flow if you want to and when doing maintenance you can stop the back flow as water is syphoned back to the tank when the pump is off. Which brings up a very important item, make sure you drill a hole in your return just below the water line as a syphon break so your sump does not flood during a power outage due to water syphoning back through the return to your sump. Make sure you leave enough room in your sump for water to flow back through your return in a power outage until the syphon break is reached.
I would use a bulkhead for several reasons. It's easier to make changes in the future, cleaner looking, less of a chance of a leak, ect.
Use a 1" PVC on the syphon to sump and a 1/2 PVC for the return sump to tank.
In the sump yes the dividers are to reduce bubbles. and the more flow the better