Would you say the same for 33's x 12.5?What size rim? as the offset will depend on that.
For a 7" rim, I would go around -13mm
For a 8" rim, I would go around -25mm
For a 10" rim, I would go around -44mm
In each case the back of the wheel will sit in the same place, its only the size of the dish that will increase.