Re: B8HS-10 plug
The -10 suffix on NGK numbers is the gap in millimeters. That plug is pre-gapped @ 1.0mm or .040 inches.
The rule of thumb on air cooled 2 strokes is to run the coldest plug that you can. The plug is only too cold if it fouls easily. if you can run an 8 or 9 heat range in NGK without fouling then use that. If the 8 fouls after a couple of tanks of fuel, step up to a 7.
A cold plug does not cost you power, but a plug that is too hot can cost you an engine.