well I tried explaining you yesterday, but it seems like you don't really get it!
Here is a picture that will help you understand the offset.
You measure the offset from the centerline! So even if the y2k wheel has a bigger offset then your stock wheel it still comes more outside then the stock bcoz. it's 9,5".
As I said, I wouldn't go with a too thick spacer. Take the min. needed to not rob on the inside or just measure and see how much you can go to the outside. You better roll in the lip like that you can go wider!
greetz
Vadim