Im'e an Ammeter ,But I often do this .First i get a race tune with old plug ,then fit new plug of correct heat range ,,run a few laps to get back up to temp ,and allow the fuel additives to coat the plug ,inspect and store for A Main .The previous comment about heat cycling makes sense also . Plug life is governed by the amount of Cadmium coating the filament ! This is because when Methanol comes into contact with certain elements ,incl Cadmium Spontaneous Combustion is achieved .The amount of time the plug is exposed to methanol governs the life of the plug . Therefore the higher the RPM the shorter time span .Rich Mixture ,short life ,plug wet after shut down ,ie stopping engine by exhaust,which leaves the combustion chamber wet ,short plug life .
this is provided youre tune set-up is spot on