Well pump output yes. It should be on the pressure tank, well pump control and/or with the paperwork you got with the well installation. You are only going to get 5-7 gpm with this system. If you have a well that is producing low output (gpm) the gpm through greensand system will be even less. As Gary stated the sfr through the "valve" alone may be 18gpm but not the the media tank with 1.5 cu ft of greensand.
RJ