I am trying to obtain the true size of a form on screen in millimeters.

I am testing using a borderless form, with:
Width = ScaleWidth = 5670
Height = ScaleWidth = 5670

So using the value of 567 twips per logical centimeter, I expect the form to measure 100mm on the screen,however it appears to be around 81mm?
What am i missing here?
I am running at 96DPI on Win 7.