How far is it to the gas station? (Don't bother answering cause its more complicated than that...)

Measure them at home, then drive to the gas station & measure them again. Now you know how much they increase from driving that short distance. That's pretty easy so try it again in summer. The amount of increase might be different when it's warm outside.

Then like you say, add 2-3 psi to whatever you got. Then next morning (cold) check to see whether this method works OK.

You're already better off than most people. You see cars driving around where 1 or 2 tires look like they have to be about 20 psi down.