It depends also on how much you care about getting the max amount of heat out of the solar panels versus pump energy use.
I recently had my solar system reviewed by hot sun industries, and they felt that there were just a few parameters to keep in mind when determining a speed:
a) if you have multiple sets of panels across various sides of the roof, try to limit water in the colder roof sides so that they all have even heating (one of them shouldn't be tremendously hotter than the other).
b) start by finding the lowest speed that moves water onto the roof and see if air gets evacuated from all the panels, if not...bump up 50 rpm, shut off pump and let water drop from panels, and try again.
c) When you get to a point where all air is getting out of the panels, make sure it isn't taking too long....ideally all air removed from panels in first 5 minutes, but up to 30 minutes or so is fine...you need to take temp readings on the panels themselves rather than just watching the return valves (although the valves are helpful). Panels that don't have full water flowing will be much hotter than panels that do. To get an idea of what that temperature is, take an ambient reading when pump is not running.
d) find out how hot the panels are or put something on the pipes returning to the pool from solar and see how much the water has been heated. It should be at least 2 degrees warmer than the pool and not more than 8 degrees. The greater the differential, the less efficient solar heating will be as energy will be lost to the environment(air/wind/etc) rather than going to pool. On the other hand, if the value is too low...than you might have issues with the solar actually cooling the water on non-optimal days. There is some temp loss in just moving water up to the roof and back and the heating has to make that back and then some extra to making heating worthwhile. Thats why you should always try to see at least a 2 degree gain.
e) try to measure all the above between noon-2pm on a sunny day to get the most accurate results.
f) check solar threshold settings in intellitouch/whatever automation you are using
g) make sure the temp sensor on the roof is positioned correctly (there are numerous gotchas that can significantly impact solar performance if it is not setup right).