50,
Personally, I like to set my lowest Speed/GPM to what it takes to close the flow switch on my SWCG.. For me this is about 1100 RPM.. I then add 100 RPM just to make sure ,and call it a day. From my point of view, there just no sense running my pump if I'm not making chlorine.. I don't care how much, or how little, water is flowing as long as the SWCG is making chlorine. I just adjust the speed down until the flow light goes out.. Then slowly increase the speed until the flow light just comes on, and then add 100 RPM.. You could do the same with GPM..
In your size pool, your IC40, at 100% output, will produce 12 ppm of FC, when run for 24 hours.. So basically .5 FC per hour (at 100% output) In the summer, most pools use about 3 ppm of FC per day..
That means at a minimum, you will need to run your pump for 6 hours. If you drop your SWCG's output to 50%, it will take 12 hours to produce 3 ppm of FC.
Obviously the amount of FC usage depends on the amount of sun and the level of CYA.. But this is a good starting point. Keep in mind that it is much easier to start high and reduce down, rather than to start too low and try to catch up.
I try to keep my pool at my "target" FC level or slightly higher.. I do not try to keep my FC between the minimum and my target.. I look at the minimum as a cliff that I never what to fall off. There is just no downside to running a little hot.. per the saltwater section of this chart..
FC/CYA Levels
Thanks,
Jim R.