Setting scales depending on vport scale

Does anyone know how to set the dimscale variable according to the scale of an active viewport. i.e. making a viewport active which has a scale of 1:50 sets dimscale to 50. If I then select a different viewport with a different scale factor it will again change the dimscale variable?

(Or does anyone have any code to do it...grovel..grovel..)

 

I rarely work with Dimensions, but here is I can get you!

make the viewport selection here, here there is only one.
Set objView = collViews.Add("TEST")
objPVP.Display (True)
doc.mSpace = True
Dim tmpdim As AcadDimension
tmpdim.ScaleFactor = objPVP.CustomScale

of an active viewport. i.e. making a viewport active which has a scale of
1:50 sets dimscale to 50. If I then select a different viewport with a
different scale factor it will again change the dimscale variable?