Sorry guys if this is obvious,but I just cant find the solution to my problem
I have a diy CNC router I have made and I want to be able to design parts to cut on it.
If I draw a square using the tool and accurately size this to 50mm x 50mm
I need a slot 3mm high and 10 long in the side of the square,so I create another rectangle with these dimensions and drag it over the square
Then select both objects and click path then difference and my shape is created,but the size changes from 50mm x 50mm to 49.997mm x 49.997mm
I have tried stroke and fill off and have it in wireframe view
Also unchecked the scale stroke width button
What am I doing wrong ?

Cheers
Jason

Possible related to the bounding box you use, Inkscape Prefrances -> Tools. Click on Geometric bounding box.
It is also possibly due to the conversion between mm and px (the format that the sizes are stored in), and floting point math rounding errors; not much you can do about that...
Although you are only talking about an error of 3 microns (half the width of a human red blood cell), so unless you are doing somthing with exceptionatly small tollarances, I doubt there will be much of a problem.

Have you tried converting the mm to inches (or vice versa) for ALL of the boxes first, then do Path, Difference? I suspect as Tomb above said, it's not adjusting well using both.
dee