Frank,
Your definition of offset is correct. In fact, you're right on all accounts.

The following is pertaining to early cars:
You can correct 55mm offset wheels with spacers. But if 15mm wheels rub on the outer fenders, there's not much you can do except roll the fenders, or pull them out in some way. In you're case, you could reduce the 2" spacers and achieve the correct spacing if you had rubbing problems.

Danno has a good write-up on his website about this.

<a href="http://members.rennlist.com/951_racerx/" target="_blank">http://members.rennlist.com/951_racerx/</a>

 

I recently put the 2" spacers on my rear wheels, and I move the 1.25" spacers to the front. These are working quite nicely with my 53mm offset, 7x15 phone dials. I haven't tried anything as hard as autocrossing yet, but so far no rubs.

Mark