I took a new 2015 20" rear wheel (so it matches the fronts) and had a company that does wheels widen it 2 1/2". To keep the same offset, I added a 1 1/4" spacer and the wheel is centered. It's a 345/25/20 tire and it has less than 1/4" clearance to the rear coil spring, so depending on what you are using for a spring, a 12" wheel may be pushing the limit with a 345 tire. It SHOULD be ok with a 315 though (but I'm not making any recommendations on that)
Thanks @funinthesun. So if I'm following it correctly, the end state is a 20x12.5 wheel with a 345/25/20 tire. Surprised it will take a 345, but nice to know. With the spacer you used, it kept with the same offset too. Now question is, I'm fairly sure I'm measuring an offset of +42 on the stock rim. I see a lot of people talking about a +45. a 3mm difference may make a difference if you are that close to the suspension.