well if u must have a taper ,makes for less critical setup/tuning, then 1/2 degree.but the outer edge should be flat at least 1mm it will trap unburnt fuel which prevents seizure. esp if it going to be the whole distance. 2-3 is waaaayy too much. unless its only the middle 1-2mm,but with all those calculations it starts getting complicated. then u gotta close the bubble down to get c/r back up.. lots a numbers lol