Typically most gaps are .030" and if a person didn't know- you could probably get away with that. On common exceptions- most Kohler CV singles are .040" and Briggs 12" Inteks have a desire to want to be set at .020" on those w/ hot restart problems. If there is reason to suspect otherwise- best look in the owner's or service manual.

IMO depends on how strong ignition system. older breaker points system I say .015 to.017. New systems .025 about normal. I would say .030 maybe too big IMO due to wear, the gap should get bigger over time. Not big fan of Champions but in small engine world about all you can get most of the time.

I think the best rule of thumb is to follow the manufacturers recommendation. All major tractor manufacturers have their manuals on line.