ATLASRECTS-7818 : Clean up the math in AlignmentErrorTool
This merge request cleans up two floating point computations that were performed in a clumsy manner in the past version.
The cleaned up computations should have the same results as in the previous version, although a small numeric error is likely to appear.
This is a follow-up MR from !67056 (merged) containing only the math clean-up discussed in this comment.