Yeah, you need to use a chrono. But, to get a general idea, you could call Martin at Airguns USA and ask him what the usual set point is for the reg in that rifle. You will probably not be able to go much below that before you see some velocity drop.
Typically around 140-145 bar. To be safe, probably refill once it crosses the 150 bar point on the gage. You can hear the audible change in sound if you pay close attention when it goes 'off regulator'.