Determining crank angle from piston position is not as simple as determining distance between points on an arc, or even determining the vertical (in line with bore) component of some degrees of rotation. You would really need to enter the connecting rod length in your calculations somewhere as you've got a rotating crank joined to a straight line slider mechanism. For example, make a careful sketch, or do some calculations, and you'll see that when the crank is at 90 degrees, the piston is not at mid stroke, but somewhat below that point.

A degree wheel starts to look pretty straight forward.