Simple answer is you can't calculate anything based on the vague setup information they gave you. With any kind of heat exchanger you get the maximum heat transfer at the maximum temperature differential. In the extreme limits, no flow would be a hot panel and at full flow a panel at pool water temperature. In the first case you get 0 BTU transferred to the pool because the flow is zero. The last case would result in the maximum BTU transfer to the pool but may restrict the flow unnecessarily. A BTU is the amount of heat needed to raise one pound of water one degree F. I would set the bypass valve to divert enough flow to the panel to result in a few degree F rise at the solar panel outlet in full sun. Nothing critical about this. Just using some numbers as an example with no idea how close they are, if no flow results in a panel temperature of 150 deg F and the water is at 75 deg F, you have a 75 deg differential. So what if the outlet is now 80 deg F. You still have a 70 deg differential so the efficiency penalty isn't much at all and you still maintain a good flow. From a pressure rise standpoint, see what the rise is with the bypass fully closed and that will give you an idea of the flow resistance the panel adds to the system. The lower the resistance the more flow you can put through the panel without taking a significant flow hit. It's all voodoo and whatever works best.
I know I rambled but I hope this helps.
Al