Found this while doing a lot of web searching, very useful IMO.
According to info I have found Benjamin Discovery has a 135 cc tank so using this calculator a Guppy tank ( 17 cu. ft. ) starting at 4500 PSI and toppping of Disco from 1.2K to 1.8K should be around 51 fills down to 1.8K
Airgun Calculators by Lloyd Sikes
Chart here list Disco as being 126 cc. , many others list also
http://www.airgunsofarizona.com/Tanks/GunVolumes.html