I do the pinch test to get a good 'ball park' setting for the highspeed needle. I then pick up the plane and point it straight up and verify. I do this before my first flight.

After that, each subsequent flight, I give the line a quick pinch to make sure I didnt bump the needle/it didnt move. You can get a feel for the RPM change and what it will sound like and its fairly easy to repeat the line pinch test accurately.

You should very rarely have to ever change your lowspeed needle once you have it set at a good setting.