You really should be looking at a turnover rate of 1.5, not 1.0. I gather you bought a multi speed because you have an attached spa since there's no need for it otherwise really. Those pumps have a maximum flow rate of 174 gpm under ideal conditions. What I would add some concern here, is over if you've added a fireman's switch into your heater's electrical as there is a timer involved here. That heater needs to shut down at least 20 minutes before the pump does, to allow it to cool properly. As for your hi/low question, you'll save on low and since those pumps are programmable, you can choose your own setting and displace the default factory settings. You'll also increase the life of the pump of you choose to go low over a longer time frame per day. Personally, I don't like to see any pump on a timer. It hammers the start capacitor on every start up.To get an approximate idea of your gpm, you need to figure out the head involved. It's not going to be entirely accurate though since that will be for pump only and assuming a straight line back to the pool, without taking into account filter and other equipment resistance, not to mention 90 degree turns but it'll be accurate enough. If you can get your turnover to 1.5, that little extra bit in electricity will save you much more in chemical costs if you keep your water balanced properly. Half speed will get you there easily.
