Did you replace your struts with stock ones? If so, it doesn't make sense that your camber would be so negative. If you put in performance ones that ended up lowering your car, then it makes more sense. Anyways, what you described is really the only way to fix negative camber at the rear wheels. But you may want to add more than one washer under each bolt. If memory serves me correctly, stock rear camber is supposed to be something like -0.3 +/- 0.5 degrees. A washer/shim that's about 2-3 mm in thickness will correct for ~0.25 degrees of negative camber. So 2 washers will probably bring it a little closer to factory settings. This the setup that I have on my car right now.