When a flat piece of glass is slumped, it is bent over a mold. This slightly reduces the effective or entrance aperture. This calculator displays the precise effective aperture, needed in CAD applications. The edge also bends outward, creating a slight angle. This edge angle is also noted. The sagitta is the spherical sagitta, the depth at the center of the mirror. This differs slightly from the parabolic sagitta, the mirror depth necessary for a particular radius of curvature for a parabolic primary.
The equation is simple: effectiveAperture = mirrorDiameter * focalRatio * 4 * Sin(1 / (4 * focalRatio)). The '1/(4*focalRatio)' is half the angle that the mirror subtends from the radius of curvature and is also the edge angle from the mirror center line.