And look at the charts..you're looking for PAR vs Distance for LED's..
You'll see the Finnex Ray II 24" listed.. at 20" it says about 50 PAR.
Low light - 15-30 micromols of PAR - CO2 is not needed, but is helpful to the plants
Medium light - 35-50 micromols of PAR - CO2 may be needed to avoid too many nuisance algae problems
High light - more than 50 micromols of PAR - pressurized CO2 is essential to avoid major algae problems