start angle and end angle of an arc in UCS

[InnerSoftCAD]

i want to draw an arc by three 3D points (not planar points)

so, once the points are selected, the UCS is changed to fit the 3-points-plane

then i get the center of the arc, but can't get the right start angle and end angle

the addarc method needs the center point in WCS, that is not a problem, but the right angles does not fit any Coordinates System (including the WCS)

just draw an arc programatically in any UCS using startangle 0 and end angle PI; these angles will not fit the UCS and will not fit the WCS: the line from center to start point, is not parallel to the X-axis of WCS and is not parallel to the x-axis of UCS

so anybody knows which is the rule?