painting fastback

Amount used can depend on color, some take more coats than other, single stage. Some colors are more translucent.
How the gun is set up means a lot also. A lot of material can be wasted.
Using the correct color sealer makes a difference.
On a metallic, that final coat is a mist to lay down metallic, which of course, takes little.
One that size car, 1 gal. would do it for me, and a very small amount left over. 2-3 medium coats. Jams, under hood and trunk lid, trunk floor just 2 coats. I don't shoot the bottom of car nor inside floor pans with topcoat. Some (off brand) cars , the trunk floor is a spatter paint not topcoat! ha But, I always want some paint left for touch up. But, at the price of topcoat, I don't want to pay for an extra gal. if not needed.

I have used TCP Globals (DTM) primers, etc. and they change their formula from time to time. I liked them better years ago than now. I much prefer SPI primers. Much.

I agree with Harrison, I have had good results with TCP Global's single stage acrylic urethanes topcoats mixed with their old PPG formulas. I admit I have never shot anything with the "$1200" per gal topcoats or a $500 gun. I am just a DIY "duffer".
If I use say, PPG mixed here local, I want them to be sure to print the formular used on the can!