Your dad was kinda right, in fact that's what the machine shop will tell you when you can see light through the valves that have "cut" seats, the burr wears off pretty quick and then you still have a nice sharp edge on the seat, usually doesn't work that way for lapping it's usually only getting worse. I just noticed that you do some Arduino as well
I had a pretty neat setup with one Arduino taking readings in the E-box linked to another on the dash with 2 OLED screens but don't use it much any more. I also made a pretty cool Arduino based fuel pressure gauge
Are you sure that your spark tester isn't marked off in thousands of volts? mine measures roughly 20mm at 30,000 volts. A really big problem with these coils is actually the spark plug boot, if you squeeze the end and can see any cracks it's a problem.