There are 3 sets of "point" fields in the shape editor infobar. The middle one always shows the x,y co-ordinates of the selected point. The other two (left and right) show either the x,y co-ordinate of the previous or next bezier control points (when the path segment is a curve) or the angle and length values to the previous or next points (when the segment is a line). It should be very easy to select the point you wish to measure from but there is an issue that does make it slightly more complicated. When you type in new values for angle and/or position then it actually moves the selected point rather than the next or previous one so you need to make sure that you select the point you want to move and then change the correct set of values on the infobar.

The angle field is measured from a horizontal line to the right (+ve x direction) in an anti-clockwise direction. The value is always displayed between -180 and 180. This is the standard way that angles are usually defined in any mathematical context. There is no way of changing this currently.

Gerry