Wheel offset question

[jonny1505]

I have a pair off weds wheels they are 18 x9 +21 3 inch lip. I'm changing to a 4 inch lip what offset will they be . 18 x10 -4 ?

[Keith_200SX]

To my knowledge the wheels will become 18x10 et9 or so. By widening the wheel another 24ish mm you only subtract 12mm from the offset!, I'm open to correction!