I am installing a Lyman reveiver sight on a Marlin model 30 AS. Two questions: 1) am I correct that to remove the factory rear sight I should drift it FROM left to the right relative to looking down the barrel from butt to barrel? 2) The factory front sight is 3/8-inch high bead mounted directly in a barrel dovetail slot (no barrel ramp). If I measure from the rear of the current fron sight to the front of the new receiver bar the distrnace is 21 1/2 inches. Is there some formula that will tell me if the existing front sight will zero at 100 yards or is it necessary to do a trial and error shooting session?