just get a low offset rim, don't be messing about trying to calculate with spacers etc. As mentioned before, 9.5 front and 10.5 rear et 20 is perfect, try and find that. If the rim in thinner, i.e. 9.0 and 10.0, you need to drop the offset to keep the same stance. The rim will be half and inch thinner, so thats 1/4 inch less poke on the outside, so you need the offset to be 1/4 inch lower (6mm). 9"/10" et 12 works well also