No need for an input guage. Don't bother.
Doesn't matter what you feed that Fatty... you can't hurt it. It can handle whatever pressure you throw at it. All you want to know is the OUTPUT pressure.
"1500 psi max input. Handles 3000 psi without damage." If you're using HPA then you're only putting around 800 PSI in it and with CO2 from 800-1100 PSI. So it doesn't matter.
Buy it... the Fatty is the awesome.