You pinch the line just before where it goes into the carb.

The pinch test works at all throttle settings (to tell if rich or lean) but you are primarily interested in setting the needles at high and idle as very few carbs have intermediate settings available.

If you pinch the fuel at the low end and see no change you mixture is probably very close. then just make sure you have good acceleration.

You aren't going to "kill" your engine. Remember you can always undo whatever you have done or go back to factory "start" positions and begin all over again