[Click on each picture to embiggen]
Using SpaceClaim 2010 and given the arc highlighted below-
I use the Trim Away tool to truncate the arc at the short horizontal line and all appears to be well. The same diameter (to 5 decimal places anyway) is reported:
However, when the two screen shots are overlaid, it becomes apparent that SpaceClaim is having difficulty in drawing the arc accurately (or precisely?):
The high level of zoom needed to work on a small area of a large object still causes issues and has apparently not been addressed as of SpaceClaim v2010.0.0.07275. This situation is a time waster as it introduces an element of uncertainly as to whether or not a design is correct or not. Really a workflow killer when one has to investigate further because something looks amiss or out of place during a quick glance.