The chopping would be done while setting the main at the beginning.

The plug colour gives a very good idea of what's going on inside the engine at the exact rpm you whacked the throttle closed and hit the kill switch

Make notes about engine feeling, rpm etc while your setting up and take a note of the plug colour. This'll help you set up faster in the future.

Don't be too scared to change a couple of things,. but if anyone's unsure of whether to go richer or leaner then go richer. If it makes the problem worse then just try going the other way instead. At least you'll be safe.