AutoCAD Map 3D :: Determine Unknown Tangent

Sometime in the past, I've drawn an arc, probably using an old deedplotting program, and imported it into Map3D.

I now have a new parcel that I'm plotting, and need to obtain the tangent of an arc on the parcel's perimeter.

Typically I draw a line, as close to the tangent as I'm able, and then toy with the bearings until I get it as close as I can.

This is a counter-productive means of finding the tangent bearing of an arc, and I'd really like to find a better way of doing this.

Is there a succint manner that I can pull a tangent line off an existing arc?

Using osnap this afternoon I thought I could do it, with the extension, but in testing it later, I found it was off by a good 2 or 3 degrees. While indeed a minor variance, I'd like to find a better way.