The suggested method is fine for WOT but doesn’t cater for partial throttle conditions. Changing the plug grade to adjust for a fuelling fault isn’t as good a solution as addressing the fuelling. Whilst this is possible with PCV, Dobeck etc not a lot can be done with the standard setup (unlike pre l/c Triumphs etc on which the fuelling map can be adjusted throughout the range using free software). Ideally the fuelling should be spot-on before adjusting spark plug heat range IMHO. Ultimately though, these days it is by no means as simple as it used to be reading a plug. Fuel containing 10% ethanol will differ from that with none in it in terms of plug colour.