I have a #20 tank that is at about 750psi. I am wondering when I should exchange the tank so that I dont run out one night with no access to another. I know a second tank would be ideal I force carb and serve at 12 psi. Have carbed and served 60 gallons with this tank. I have only forced at 30psi with 2 or 3 kegs. When does everyone decide its time to exchange/refill so that you know you can continue serving? Thanks