Hey Brad, this is a picture explaining the different terms and referring to the stock JK 7.5in rims:
This is a diagramatic representation of your new wheels to your stock wheels:
And this is the raw data comparison - as you can see the outer edge of the tyre will be 3in further out than stock. To keep 285's inside stock guards totally an offset of around +20mm is pretty much all you go.
The last two pics are from the calculator on
http://www.rimsntires.com