I do not understand exactly your problem.
You have the MMPerPixel Scale factor.
In this factor, are the pixels those of the bitmap or those of the view showing the bitmap ?
Do you want to have the dimensions of the item in mm on the screen ?
I mean, if the width of in item is 10mm and you measure it on the screen it should be 10mm.
In that case you can use the code
HERE, where you get the real pixel size in inches.
Can you post a small project showing the problem.
Instead of 100 / 100dip you could use xui.Scale