Trial and error is right. Just think of all the variables that effect the amount of air coming into the cylinder before that intake valve touches the seat.Carb.size plenum volume,runner distribution variance,runner entry shape,taper&size,valve size,lift rate,seat angles,port shape& flow,chamber shape,valve shrouding, piston acceleration rate&velocity,brand of beer consumed while assembling the engine,etc.etc. It boggles the mind.