4 degrees is probably the most common setup and either that or seven degrees should work, perhaps depending in part on when you plan to mostly use the RADAR at idle/trolling speeds, or up on plane.
I actually got up on my top and used an angle finder to get level while running. I bought a fixed 6 degree mount and still wound up adding another 1/8" of shim to get the radar picture I was looking for. I'm about 8 degrees down angle now, running at normal speed. I want to see what's in front of me while running at night or in fog out as far as I can. My unit picks up bouy's, kayaks, big boat wakes, and even mooring balls, so I'm happy. I think trial and error will get you what you're looking for.