I don't know how the person who is painting your car paints but 1.5 gallons is lots of base coat. Reduced it should take 1.5 US quarts per coat maximum which means that you do in fact have enough for 4 full wet coats of base. PPG Deltron covers well and the fact that you are going over top of a dark grey/black primer and the color is dark, it should cover in 2 coats. That being said, if you don't have a mixing system it's always a better idea to have more paint on hand than you need, there's nothing worse than running out in the middle of a paint job. Just a tip...don't reduce all the base coat...reduce what it as you need it and when your done make sure it's sealed up real good and tight...it'll last for years then.