You can top a 75cf off with a Hill but it will take a lot of stops and starts. Hill compressors are made to fill guns rather than tanks. You should refill your tank when you reach the level in your tank that is the same as the pressure level required to give you a full fill in your PCP. For instance if it is 3000 psi on your rifle, then refill when you can't get a full fill from your tank to 3000. I usually refill my tanks when they reach 3600 psi since I have several PCPs that fill to that level.