How to measure in tight spaces:
Center the rudder, measure from the center of the quadrant or tiller to the bolt or cable. Mark where you measured with a grease pencil.
Tack a stick in place on centerline just ahead of the rudder or quadrant. Ideally it would be a plumb bob, but normally there isn't room for such nice things. Rotate the wheel
to the lock. Measure from your mark on the tiller to the stick. Measure from the stick to the centerline.
Now go get a piece of cardboard and lay out your measurements and set a protractor over the top.
Draw a centerline, draw a perpendicular line. Pin a hacksaw blade to the intersecting point.
Measure out from the pin, the tiller or quadrant length. Draw a half circle. Measure along the centerline to put a mark where the stick or plumb bob was. Measure from the stick along the circle the distance you measured to the mark you made on the quadrant. Connect the mark to the spot where the hacksaw blade was pinned and you are done.
Set the protractor on the line you drew perpendicular the centerline and measure the angle.