I don't know about 3.1 since I use 3.5 but try this; as soon as you load up zbrush open up the default sphere and then upload this to Shapeways as a test. This will give you an idea when you later create a model (tool) and compare it to the sphere on screen. This is not an exact method but is not bad. I too have been struggling with getting exact measurments.
(And also with exporting to stl directly, but that is another matter and doesn't apply to zBrush 3.1 as I recall)Glenn ------ My Website Third Dimension Jewellery