I needed a 3/4" riser, but that was for a different sight.
Mount your Fast Fire on the rail, measure up from the top of the rail to mid-point of the glass on the dot. Subtract this number from the same measurement to the center of the aperture rear sight. This will get you real close. That is how I did it and my sights co-witness perfectly.
It helps that the iron sights are sighted in to begin with though.