Totally agree, MAS985! 8 to 10 psi seems to be a reasonable target for most (though I'm not sure if there is a difference between "permanent" and the more flexible "above ground" panels) solar installations. Maximum flow through the panels without exceeding there designed pressure limits is best.

What I was trying to make clear, but maybe didn't, is that you can use the diverter to hit that sweet spot - and then no more - even if your pump is capable of over driving your panels. If the pressure drops immediately when you start to open the diverter, your pump isn't "overdriving" the panels at all (sounds like this is happening in your case) and you can just leave the diverter completely closed.

Maybe a better way is to start with the diverter fully open and close it down until you see a 7 to 10 psi increase (or until the diverter is fully closed). Closing it any further at that point would lead to more pressure, but no real flow increase.