Very simple and no need for all the baffles and such in freshwater.
The water must always go into the overflow so the level in the tank can not change. Get it?
It changes in the last section of the sump,which IMO
should be the largest section to reduce top off needs.
Many build their own overflows if they know how,or you can get some from BRS , but Eshops has a new one out that I really like...
ESHOPPS | Eclipse L
^^I have one of these in small for a 29g build..^^
You want an overflow that can handle way more then you will need.
The return pump has final say on flow as long as the overflow can handle it.
I run jeboa 12,000/15,000 on my 3 big tanks. The pump is digitally controlled so I can easily adjust the flow.