It's -12mm....not degrees.
The important thing to check is the backspacing. If you ever intend to run a Rancho lift, then you should buy wheels with the correct offset to clear the knuckles. I know that 17x9 wheels with a -11mm offset work nicely with the Rancho....and thus yields approx. 4.5" of backspacing. But 10" wide wheels (and also the larger 20" dia.) will not fit the same way. Each wheel is different and your best bet is to verify the actual backspacing with the manufacturer. Unlike offset (which requires some math and assumptions)...backspacing is an absolute real measurement. It's fool-proof.