The owners manual for my '64 says "it should be about 1" below the ring inside the filler neck" and goes on to mention checking when cool of course. This is on page 17. I can scan it if you need to show the youngen but he should just listen to your experience and take note.
Also let him know that if overfilled it will find it's natural level after driving and shutting down it may spew a little then stop if all else is good.
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