Trouble with VIEWSIZE

I have a routine that will open a selected xref and zoom in that xref to
match the screen of the host drawing. I use VIEWSIZE along with ZOOM Center
to get the screen match. This works fine if the xref is selected while on
the Model tab. However, when in a locked pspace viewport VIEWSIZE is
constant no matter how close or far I'm zoomed to the viewport. I can be
zoomed in to see only one square inch of pspace but VIEWSIZE will still be
the same for the viewport.

My question is how can I determine the true VIEWSIZE or zoom scale factor
when in pspace? I can get the zoom xp scale from dxf codes and the VIEWSIZE
of pspace but I can't figure the relationship between them to calculate the
zoom factor for the xref.

Thx
Josh