If you're doing calculation and need the correct number of significant figures, it's never a problem when a conversion has more than necessary. Otherwise, you can get rounding errors.
It would be useful it you could simply type the number without a keyword, and without a space on the unit ie.
4.1mm
Instead of stating that no destination or defaults set, it would be useful to list the most common conversions (ex.. in the US its very common to convert mm to inch, whereas mm to cm is rather easy for a person). Check out the default setup for Spotlight, I think it lists common conversions, with no keyword.
BTW, I only critique because the workflow is very cool and useful, thanks for it.
