The Rotation property of 2d entities (Text, BlockReference, ...) is the angle in radians measured CCW from the X axis of the entity Object Coordinate System (OCS). The origin of OCS is always the same the WCS origin.
If the entity construction plane is parallel to the WCS XY plane, the OCS is equal to WCS.
To learn more about OCS, have a look at the Developer Documentation in AutoCAD Help > DXF References > Advanced DXF Issues > Object Coordinates System (OCS).
So, to get the rotation of a Text (or MText, BlockReference, ...) about the UCS (assuming the current UCS plane is parallel to the entity construction plane), you have to substract the angle between the OCS X axis and the UCS X axis from the Rotation property.
If the UCS plane is parallel to the WCS plane, you can simply get the UCS rotation this way:
If the UCS plane is not parallel to the WCS plane, you have to get the OCS corresponding to the current UCS first. As said upper te OCS origin is always Point3d.Origin and the OCS Z axis parallel to th UCS Z axis.