I would imagine a lot of guys here have feeler gauges around and have gapped a spark plug or two in their lifetime.
You can show a guy your multi hundred dollar tool for measuring FP protrusion from the front of the bolt or...
Explain to him how he likely already has all the stuff he needs to measure the same thing from the back.
https://i.imgur.com/cBOoyAZ.jpg
That is the gap you measure while holding something flat against the face of the bolt and pushing the FP forward until it stops against it.
Not hard to do and does not require a $500.00 specialty tool to measure.