How wide the edge is from the bevel's shoulder to the actual edge is going to depend on the thickness of the blade and the angle at which it is sharpened.
It can be different for every knife. Typically for thinner blades it will be very thin. It looks like on your knife it was possibly less than a mm.
And yes, if you think of a plane passing through the blade from heel to tip perpendicular to the spine, centered on the edge, and parallel to the sides, that would be what the angle is relative to.
So raising the spine to get that plane 20 degrees relative to the stone would give you a 20 degree bevel on that side. Just remember that exact angle aren't important (unless it's 90 degrees lol
) but consistent angles are.
You are sharpening by hand, so some variation is completely natural. The key is to minimize it as much as possible and the only way to do that is practice.
The angle finders (I think) are for use with a system like the Edge Pro so setting it up for a particular knife will be faster and easier. They aren't needed for hand sharpening... unless you're OCD about it... lol