Can anyone advise on feet of head required to push water through PEX [poly] tubing, 1/2" ?

I want about 1 to 3 gallons per minute through EACH of [4] 300 foot loops in a slab. There are 4 ball valves that can isolate each zone, so at times only one or two zones are open. When all 4 zones [1200'] are open flow as low as 1 gpm or less should be alright.

I am looking at Taco and Grundfos pumps in Grainger [open loop - need bronze or SS head]

Maybe the three speed motor from Grundfos to allow for adjustment?

Simplified question; How many feet of head required to push water at 2GPM throu 100' of PEX?

I have done several systems before, but would like to not simply guess at the unit or opt for a high head and throttle the pump. These are impedance protected so overload is not an issue.

Thanks for any math in advance!