Calculating energy savings & more using a variable-speed pump

I've got a variable-speed 1.5H pump. It runs between 600rpm and 3450rpms. Right now, I run it a minimum of 8 hours a day @ 2750rpms -- about 80% of capacity. And anytime we're in the pool after 4pm (when the timer shuts of the pump), I'll run the pump until a few hours after everyone gets out of the pool. Since this is a new pool, we're in it every evening, so I probably am running the pump, on average, about 12 hours a day @ 2750rpms. Lastly, the CAD drawing for our pool shows that the turnover rate is 1.8/hr -- about once every two hours, which I suspect is under ideal conditions (clean filter, pump running at full capacity, valves opened up to maximize water flow). Currently, the water is crystal clear, even with a heavy bather load (thanks to our DE filter, I guess). I live in South Florida.

Questions:
- Is there a way to calculate how much energy savings I get by running a pump at less than 100%? Does running a pump at 80% speed reduce the power consumption by 20%?
- In a warm-weather climate, for how long is it adviseable to run a pump in the summer? In the (tropical) winter?
- If you run a pump at 100% for 6 hours, is it as effective/costly to run a pump at, say, 50% for 12 hours?

Thanks