simple math in custom field

[macduda]

Hi please help me understand how to reference the width of a shape in custom formula to display, without making my head explode...
I would like to do some simple math to display a custom formula in a field on a shape, but something is not working as expected.

I have a shape with a width of 3130 (um)
In a field with a custom formula I can print width as =width and it prints 3130

I run into trouble if I try to do a simple addition
=SUM(width,10) --> it displays 3384

Is there a way I need to convert the numbers to the same number format?
What am I doing wrong here?

Likewise I have a custom shape data named test value = 10 (as a float)
if I display =sum(width,prop.Row_1) --> it displays 4654

Thanks!!

[Paul Herber]

The 10 here defaults to Visio's internal units (inches), add mm after the 10 and all should be well with the universe.