AutoCAD .NET :: Calculating Angle Between UCS And WCS

A question regarding converting angles between co-ordinate systems in AutoCAD .NET.

Im creating my first little utility to form selected block attribute references into a nice neat list. The current version takes a selection of blocks and a list insert location and then moves the attribute references to the location and rotates them to WCS angle of 0.

I now want to expand the program to make it a little more flexible and allow you to form the list parallel to the currently selected UCS x axis (the i.e. and angle of 0 in relation to current ucs's).

I have managed bumble my way through and transform the attribute positions correctly to form a list relative to the current UCS but am strugling on how to work out in code what the equivlanent WCS angle value would be.

I have done some calcs using .net framework to calculate an angle between two vectors (see code extract below). But this introduces a small error into the angles (i.e. if my ucs is set rotated 45 degrees around the zed and i use the below maths, the resulatant angle is 45.0000002 when read in acad afterwards).

Matrix3d ucsCur = ed.CurrentUserCoordinateSystem;
CoordinateSystem3d cs = ucsCur.CoordinateSystem3d;

// the below seems to work by calugulating the angle between two vetrex using .net maths class but this introduce a small accuracy error. Is there a better way?

Double rotAngle = Math.Acos((1 * cs.Xaxis.X) + (0 * cs.Xaxis.Y) + (0 * cs.Xaxis.Z));
//set the attribute reference rotation to the calculated WCS radian rotation value "rotAngle" 
attRef.Rotation = rotAngle;

Im sure there is a better way built into the managed autocad wrapper using a tranform matix and the ucs vertex information but dont really know how to go about it. Any links explaining how to work with the UCS in .net.