I can't help you with your VB code, I'm afraid - I don't know enough about it. If you use an Extended Events session or Profiler trace to capture the SQL that is actually executed, that would probably provide a clue about what's going wrong. But my advice is not to do this at all. Store your measurements as mm, and if certain users want to see inches, do that for them in the presentation layer.
John
The units are stored as mm, that is root of the problem