Somebody help! I have a Bushmaster Shorty carbine with an A-3 top. There are no markings on either the receiver or the frame of the sight. Does anyone know how to determine the 300 meter setting on this thing? The manual says to adjust the setting to 300 meters, then up one click to start the zeroing procedure. Heck, my elevation drum really doesn't click, it just turns slowly to where it might stay in one spot, and there are no numerical markings on it, either! I am hopelessly lost! Thanks in advance if anyone can help.