Ok, based upon that information here are a few estimates and I say estimates because we really don't know what is happening on the suction side but from the filter pressure I can get close. We can nail it spot on if you can actually measure the suction at the pump via the drain plug but you would need a vacuum guage for that.

Anyway, here is what I have:

With the single 1 1/2" line: ~40 GPM @ 50' of head
All lines open: ~48 gpm @ 44' of head

The filter can probably handle much more than that so I wouldn't worry too much about flow rates.

Let me know if you need anything else.