2010 gmc crew cab lift tires and wheels

[2010GMCSierra]

SO I dont quite understand the Mathematics on offset wheels, I have a 2010 GMC Seirra Crew cab with a Rough Country Leveling kit, Im due for tires and Im considering upgrading to 20" wheels, Ive seen alot online with a 30mm offset, where are we going with this lol, so I was going to run 285/70/17s, but I would rather buy a set of 20" wheels what are my options?

 

[GMC Fishingnut69]

30 mm is the stock offstet. I'm running a motofab 3 inch front level kit, nitto terra g2 285/70r17 (diamiter is 32.76 inches) and Black Rihno sierra rims 17x9 (-12mm offset).
My off set make tire stick out about 2 more inches from stock.

example:http://cocomponents.com/dealer/blog/wheel-offset-explained/

 

[bamf]

contact the wheel mfg they should be able to help