Sorry for being blunt, I just know a lot of new hobbyists who insisted on going routes similar to this, trying to "balance" drains and returns using valves, and it always ends badly. I myself completely ignored all the advice I was given to the contrary when I started out, and I always had flooding problems. One guy finally told me "open the drain valves up all the way" and said "trust me, it'll just work." Well I did and only then did it occur to me WHY it works that way. Once you understand how overflows work, it makes perfect sense.
I don't want to see folks make the same mistakes I did.
Okay so I drew up a professional-looking diagram of how I would set up your system. Let me walk you through it.
Basically, you have your fuge on the left and your sump on the right. Quite right, you don't want to run ALL of your flow through the fuge. So here is how I solved that...
The blue lines represent the drain from the tank. If your overflow has two bulkheads, they should be combined into one large line PRIOR to this diagram. Orient your pipe in a horizontal run along the underside of your tank stand. Above the fuge, install a "T" with the outlet pointing DOWN. Drop down to a valve (1).
Then, extending horizontally, you can optionally install a second valve (2) that you would normally leave open ALL THE WAY, closing it only for maintenance.
If your overflow has (2) 1" bulkheads, I'd combine them into a single 1.5" line. The drop to the fuge can be reduce to 1" or 3/4", but the entire horizontal run to the sump should be the full 1.5. This means that you can close the fuge valve entirely and the rest of the drain system could handle the entire flow if you wanted it to.
With the drop to the fuge extending downwards from the drain run, it will work so that only the water that you allow through the (1) valve will pass, the rest will continue onto the sump. Again, leaving (2) wide open except for maintenance will allow you complete control of the flow to your fuge, diverting what you DON'T send to the fuge to the sump.
Connecting the fuge and the sump is a piece of pipe, at least equal to the size of the drain line feeding the fuge. The fuge is drilled at the desired water level, and it connects to a hole near the bottom of the sump. This allows the water in the fuge to rise to the desired water level, then be drained down to the sump, creating and overflow in the fuge.
Finally, notice that the drain line to the sump is brought in on the far right, and that between it and the fuge inlet on the far left (of the sump), is the skimmer and return pump.
The return pump (3) is located between the skimmer and the fuge outlet. This is intentional. It maximizes the chances of the water flowing into the sump from the fuge gets picked up by the pump instead of going through the skimmer, which is less than ideal. Conversely, the skimmer picks up water flowing mostly from the drain on the right.
I'm actually working on a 120g reef system at the moment that includes plumbing very similar to this. I can take pics if needed, but I don't have my camera at home atm, so let me know if this all makes sense to you.
If you really want to do this right, you can also install a baffle between the skimmer and return pump. This will ensure that the skimmer does not process water from the fuge. The tradeoff is that you have less "reserve" water for evaporation loss, but many will argue that it's worth it.
