Below is the specifications sheet from our current catalog with Co-Witness Height information.
http://www.truglo.com/images/red_dot_specifications.pdf
Depending on the specific sight picture you wish to achieve, this will determine the riser height required. Additionally, the specific sight you are installing will impact this choice as center lens location is different on many models. Many factors come into play to make this decision. The style of the buttstock and the height of its comb, the shooter’s face/head position on this stock and the achieved height of your line of sight. We recommend taking your firearm to a local gun shop with a variety of riser heights available and see which heights provides the best, most comfortable and effective sight picture you require.