It can work either way I guess. I don't see any real disadvantage to getting your trigger a degree or two earlier. You won't have to adjust the sensor spacing because you just adjust it in the spreadsheet.

This is one feature I did think of. There should be no sensor adjustments at all. You fix your sensor permanently, as close to where you want it as possible. Then adjust through the spreadsheet.

I'm not sure about the spark issue. I believe I have the delay set for 1000 uS, so the sparks should be 1 ms apart or 1/1000th of a second. You shouldn't be able to hear then except as a buzz.