@Gary ******
You already used your tourist visa when you entered Thailand. It's SINGLE entry, unless you pay for multiple entry. As soon as you enter Thailand your visa is gone. The expiration date on your visa no longer matters because it's garbage at that point.
Once you cross passport control the only thing that matters is the date stamped in your passport. That's your entry stamp. If you purchase a re-entry permit the only thing that happens is your stamp isn't cancelled when you leave Thailand, you can return on the same entry stamp. You will not get any extra time at all.
If you purchase a re-entry permit and return after the date that was stamped in your passport, you completed wasted your money you spent on a re-entry permit because that stamp has already ended.