It should be listed in the manual or you can look it up on the web. If you cannot find a reference then just use 80%. It is probably close enough.
GPM can be approximated as well by the pump's head curve. You can pick somewhere in the middle just to get an idea of the numbers you are dealing with. Another good approximation for head loss is about 3 times the filter PSI reading. Then look up the GPM on the head curve of the pump.