Offset bushings don't really work in reverse unfortunately. They have a tendency to rotate to the slacker positioning in short order, so it won't really work for very long.
You can't determine the exact travel when swapping a 42.5 to a 45 because you have to take axle path in to account, and most bikes don't move through their travel relative to shock stroke in a linear manner. So it's kinda unpredictable exactly how much more travel you'll get. You can do a simple approximation but it isn't elegant and won't really get you the answer you want.
The inelegant way is that 45mm is roughly 6% more than 42.5. So that's approximately 127mm of travel. But as I said, that's an approximation and the math will work better for some frames than others.
