The set of plug gauges that I use are Minus .0003". That means that a .430" marked gauge actually measures .4297". In other words you want minus gauges because you couldn't stick a .430" gauge in a .430" hole. Being minus by .0003" is close enough for measuring cylinder throats. If you can slide a .430" gauge in a throat but not a .431" gauge, the actual throat is somewhere between to .4297 and .4306" and you can use .430 diameter bullets.