Welome to the forum Benny!
There are many ways of plumbing this. When I plumb tanks I generally like to use a 'T' off the main return line and use that to feed the refugium. You could also use a 'T' off the overflow, but it is easier to regular gph using the return line instead.
If I was setting up this tank for a client, I would use a 700-900gph return pump and would T-off ~100gph into the refugium. I would use (2) additional powerheads in the display for additional circulation. Perhaps something similar to a Koralia 2.