I'm no help on backspacing, but I would want more than bare minimum clearance on the brakes. My dad had a set of winter rims/tires for his Tundra which sat pretty close to the calipers. Most of the time, they were fine, but now and then they'd make a racket like the world was ending. Near as I could work out, this was due to a rock or something getting caught in between the caliper and the rim. It never tore off a caliper or punched a hole in a rim, but the noise scared us pretty bad once or twice. Also keep in mind that the calipers may stick out more with new pads on them, so what barely clears now may not clear after your next brake job.