48% of all babies are born left handed, by adulthood there are less than 15%. Therefore at least 33% of all shooters will probably have a master eye problem, to a greater or lesser degree. Eye dominance can change as well, due to, tiredness, ill health, age, damage to head or eyes, or even to the type of employment, sitting in front of a computer for long periods, this could account for up to 20% of right handed and right master eyed shooters having a master eye problem. Your master eye, for a lot of shooters, does not mean either left or right eye dominant, you could have central vision, left central or right central vision, all of these problems will more than likely affect your shooting accuracy.
There are various ways to check your eye dominance, the simplest is with your left hand (right handed shooter) bring your thumb and forefinger together to make a circle, (you can use a DVD, large washer etc.) raise your hand at arms length, aim at any stationery small target and then close firstly your left eye, then your right eye, which one is on the target, do this a number of times to make sure you are getting consistent master eye information, from this you will be able to see if you have a master eye problem or not.
You can use any of the four sizes of Redeye but it is recommended that you start with the 16mm diameter and with practice you may be able to reduce the size of the Redeye being used. Occluding one eye by closing it or blocking it completely, loses depth perception, judgement of speed, field of vision and peripheral vision, Redeye gives you the opportunity to reduce any loss of vision to the bare minimum.