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in my opinion you need a 5mm lower offset in the rear(so the rear sticks out 5mm more than the fronts do) compared to the front for a flush look. so a 4 or 5 mm spacer will do the job
But if you mount 4 or 5 mm spacers they are not hub centric and they push the center of the rim out of the hub of the car and you have no hub centric support. That's why I always go for wheels with a higher offset and correct them with hub centric spacers... like in the picture below 10mm and 15mm