It depends on what your goals are . . . .
A good place to start is to determine the amount of turnover you're after (e.g. 10x per hour, 20x, etc.) and multiply that by your total system volume to get an idea of how many GPH you need from a pump.
Also, I could be wrong on this and would like to know for sure, but I don't think that there is head loss on a CL system because its all from within the same body of water (e.g. the display) so there is no gravity differential . . . again, not sure.