If you already bought the pump, connect it to your chiller and see how much water it pumps. I have a submersible pump rated at 920 gph, but only for a lift of 10 ft. My chiller coil presents enough resistance that I don't get anywhere near 920 gph through it. I get maybe 1/10 of that, which is enough to cool down 5 gallons in about 20 mins. What you get depends on the diameter and length of your chiller tubing, how tight the coil is wound, and the lift capacity of your pump in addition to the gph rating. Although it is possible in principle to calculate all this out, experimenting is the only way to be sure.