Get Low! Get Low!

Ok. I have some rubbin problems on my 89GT. I've done everything I could think of to leave to 17x10 cobra r's on the front. I know, it was stupid even puttin them on . Well I put on new stock springs and some kyb shocks and my front end is sky high and it still friggin rubs , so I'm putting on 17x9's. Does anyone know what the best backspacing is on these, in millimeters? I'm about to go with 20mm, does that sounds right to anybody?? One more thing, If I put on fms lowering springs, should i get the 400# springs or the 600 some # springs? Thanks for any help!!