Two big factors.....

Are you using a HVLP or old style siphon gun? With The HVLP more material goes on the car and less in the air so less paint is required.

Mixing ratio on the paint is also important. It’s not so much how much paint you buy, but how much sprayable paint you end up with. The ratio of thinner and hardener the paint requires can make a big difference in how far the paint actually goes. I was reminded of this the hard way the last car I painted.....I was trying a different paint and the amount of thinner and hardener required for this particular paint was a lot less than I was used to. I Barely had enough to finish the job.

I’ve been using SS Nason and SS Croma 1urethanes for the last few years and on those you need to cut and buff within 24 hours to get a good finish. If you let it sit longer it tends to haze when you buff it. I suspect the Summit may be the same way.

Just a suggestion but for a first paint job I would suggest maybe trying Nason Acrylic Enamel. It goes on nice and is pretty easy to work with. Like all the old enamels, you can drive it within 24 hours but it needs to cure for 90 days prior to buffing or waxing (the cutting and buffing window is indefinite after that).

Doing this the first time and including jambs, firewall, inside deck lid and hood, probably 6 quarts color plus the required reducer and hardener for a siphon gun (8 if you’re you plan on painting the fenders, doors and hood/deck lid off the car).

When you get the paint mix (especially if you go with Enamel and have it mixed locally) get a bucket and mix all the cans together to make sure you get a uniform color.