Planning upgrade

[TheMikey]

I've been feeling the itch to upgrade my 15G nano to a 20G long tank and plan to start in late April after my final exams are done from school.

This is going to be a big upgrade for me as my current nano is pretty bare-bones in terms of high-tech equipment.

As of right now my stats are as follows: 15G (24x10x12 I think), 2 x Koralia 1 powerheads, 200W heater, and a Tunze 9002 skimmer. For lighting I have a 4 x 24W T5HO Aquaticlife Fixture.

My upgrade plans are:

20G Long (30 x 13 x 13) - plan to drill and build overflow, more on this later
Sump (w/ Refugium)
4 x 24W Aquaticlife T5HO
Add another small heater (for backup)
Tunze 9002 Nano Skimmer
2 x Koralia 1 powerheads (may upgrade this later, but for now these will do).

Specific Plans:

I'll be buying the 20G Long from Pet Smart (as they have them for a great price) and trying to find someone local to take me through the drilling process. Afterward I will be painting the back (probably blue as I like it more than black) and adding an acrylic overflow. I will be removing the trim and cleaning this up substantially so that it looks perfect - I'm also going to call around Saskatoon to some tank builders and get quotes. I'm not opposed to paying a little extra for quality, so that's an option as well.

My fixture is only 24" long, so I will have to silicone some acrylic "braces" to accomodate the legs. This is a really powerful fixture and my clam and SPS love the light, so this won't be much of an issue for the new tank.

As for the sump, I haven't decided on a size yet, but it probably won't be smaller than 10G. If my current 15G can fit in the stand, I'll use that and just silicone some baffles in for a bubble trap.

The skimmer will go in the sump, though I'm not sure where because as a hang on it's designed funny.

So here's the dilemma: I've been scouring the internet for information about sumps and the like and haven't found a lot of information about overflows and pump ratings that have been useful.

I have no idea of anything in regards to sumps in regards to a return pump that would be adequate for a pre-drilled aquarium. Can anyone shed light on this for me? Hang-on overflows make sense to me, because they have a pre-determined GPH rate that you have to match your return to after calculating head loss.

So, the point of this first "help" post is, how do I calculate the rate my water will fall with an internal overflow so as to choose a proper return pump?

[Bloodasp]

The overflow should have a greater capacity than the return pump.