I forgot to ask two more questions... single stage or base/clear? And also what model gun are you using?
If you are spraying it in base coat a gallon will be plenty. A gallon plus a quart will be more than enough. That is ready to spray (RTS) or reduced. If my memory is correct PPG base coat is mixed 1:1. 1 part paint 1 part reducer so you will need 2 quarts of unreduced paint and 2 quarts of reducer for a gallon of RTS paint. If you are using a cheap spray gun you will use a little more paint but if you are using a mid to high level spray gun you will use less paint because of the transfer efficiency of the gun.
If you are using single stage and using a decent gun you will use less than a gallon of RTS single stage. Reason being is because single stage will cover much better than basecoat. If you plan on wet sanding your single stage you should put an extra coat on since you will sand a few mils off, depending on how agressive you get with the wet sanding. (base coat you don't sand, you would sand the clear)
Does all that make sense???