Pressure left in tank

[cfxlongshot]

I have been shooting my 202s lately and I am not sure how low I should allow the tank to go before I refill it.  I fill to the 200 bar limit which shows up as a 20 on the gun's manometer and shoot until it gets to the 10.  How do I check what the pressure is?  When I fill up the guage goes to the tank pressure and does not show what is left in the gun.  It still feels powerful even at 10.