The sharpening angle is the same as your edge angle. Also, these images were drawn using state of the art drafting software (AKA Word), so please excuse the eye pain.
Just before we start, going from a V edge at 20 degrees to a convex edge does
not create a 20 degree convex edge. Alright, let's go.
A "V" edge is formed with solid, flat abrasives. If you hold the knife at 90 degrees, and angle your sharpening stones at 110 degrees and 70 degrees (left and right sides of the blade), and then sharpen, you will get a 20 degree edge (40 degrees inclusive). (See below image)
A convex edge is formed by a belt system (or some type of system with a tension or "give" to the abrasive surface). If you hold the knife at 90 degrees, and angle the belts at 110 degrees and 70 degrees (left and right sides of the blade), and then sharpen, you will get a 20 degree convex edge angle (40 degrees inclusive). (See below image)
So, take sharpening systems that produce a V edge and convex edge, angle them both at 20 degrees with respect to the blade, and here is what you get:
As you can see, the convex edge clearly has more material behind the edge than the V edge,
with the same edge angle.
I drew the pictures to scale, so feel free to compare the angles on your screen. The red dots represent the points on the original knife blank (also at the same 20 degree angle).
I welcome any corrections or alternate views as to why this could be wrong (provide some sort of proof though).
Also, this does not take into account convexity, because that does not matter. Technically, you could apply this to a hollow grind too, where a wheel would be used, and would contact the knife at the two red dots as well. Changing wheel diameter would change concavity, but hollow grinds are a bit different, so it wouldn't be completely accurate.