AutoCAD .NET :: Measure Perpendicular Distance Between 2 Polylines?

I need the perpendicular distance between 2 polylines.  If I was doing this manually, I would draw a line perp from 1 side and extend it to the other side, check it's length, and delete it.  These are pipeline r/w's that quite often have variable width, and are not always parallel.

I am asking the user to select the pline 1st (GetEntity) to get some object data.  Then I prompt for a point on the pline where a block will be placed. The block contains an attribute definition for the width.  I check to make sure the point is on the pline.  I was hoping I could get the user to select a point on the opposite side and force that to be perp.  That would give me the width but it doesn't work.  I have looked at GetFirstDerivative and rotate that 90 but the examples I found use a certain length.  The perp length is what I need to determine.