For a 9" wheel, Method's +12mm offset is as perfect as you can get for fat tires, if you don't want that "poke" that many brodozers are running. It fills the fender wells as much as you can before adding poke if that makes since.
I am running equal to a 35x13.5R18 and the inside of the tire is slightly less than 1/4" from the UCA and zero rubbing on anything that matters. A +18 will bring the wheel in another 0.24" which should be fine for 275's. because that is 2" more narrow than mine. So, 1" less inside projection. If I have 0.25" clearance with 325's and +12, you should have about 0.75" with 275's and +18. 0mm will push the tire out about 3/4", resulting in more "poke".
The first pic shows the "poke" with 13.5 wide tires..just barely outside the fender well.
View attachment 176395
View attachment 176396