Also, i forgot to add this. When I used the dremel to shave the materials off of my #3 backstrap, in addition to taking the materials from the bottom of the backstrap to make it even with the magwell, I had to radius cut the rear down a fair amount.
Take down the width of what's left of the bottom raised portion deeper, and the first recessed portion, than the recessed portion. You will remove the raised bottom portion and the first recessed portion. Here's a great MS Paint drawing using Mr. Springer's picture from page 1.