A +35mm will work but may rub. Personally a +40 would be Ideal but with only lowering 1.7", a +35mm should be fine as long as you stick with a 245/40 tire, the tires should end up about a 1/4" in from the edge. Just do a search or look the definative section 18" to 20" wheel post, there is a lot of 20" wheels with specs and lowered, you will find you answer.
Later
steve
Later
steve