I have the same question, and talked to many, whom I believe to have adequate experience. Such as a friend who paints cars professionally, a former restorer, paint store, others on here, etc. I have gleaned that there are too many variables.....coverage of the particular product, primer color, paint color, number of coats, how much you spill, excess overspray, any boo boos along the way. Colors change lot to lot, custom mixes are hard to duplicate, even if you go back to the same place, same code, same operator. Like I said, lots of variables. Its never bad to have an extra quart. Who knows, you may need to make a repair someday. A scuff or a nick in assembly, a ding on the road. Just shake and flip the extra every month or so.
It takes too much effort to go cheap on the final step. get the best guess, buy extra. The last vehicle I painted was a jeep. Bought a gallon, exterior only. Ended up short because I managed to spill a quart along the way. I was half a quart short.
Lucky find on a desert truck. Minimal rust is a nice find, especially on a vehicle of that age. Good luck!
