AutoCAD Visual LISP / AutoLISP :: How To Get Viewport Scale

I am adapting a routine by Alan J Thompson found here for placing an outline of a paperspace viewport in modelspace.

I have added some lines for making a suitable No plot layer with appropriate linetype, and wanted to make the lwpolyline have a thickness dependent upon the viewport scale.

My paperspace viewport scales are set for printing via Zoom - 1/50XP for example to get a 1:50 scale plot (or 0.02XP)

Having selected the relevant viewport, I wanted to be able to determine the vp scale, but am unsure how to extract this information from the viewport entity itself.

The routine was as follows:-

(defun xpfact ()
(- (car (trans '(1 0 0) 2 3))
(car (trans '(0 0 0) 2 3))
)
)
--