That's entirely the wrong tool for the job. TFormVector translates a vector from one space to another, it doesn't measure distance the between two entities.
Check out EntityDistance. If you don't want to do the calc yourself, just create two pivots at the appropriate locations and do an EntityDistance on them.
Check out EntityDistance. If you don't want to do the calc yourself, just create two pivots at the appropriate locations and do an EntityDistance on them.