just measure from where the wheel face mounts to the hub, from that surface to the inside lip ( the side of the wheel closest to the suspension) for this example sake lets say you get 3 inches. you will be widening them to 9 inches wide, therefor adding 6 inches past the hub. zero offset would have 4.5 inches each side of the hub, but we started with a measurement of 3 inches of back space so find out the difference between zero and where the hub actually sits and that is your offset. in this case it will be 1.5inches or what ever that is in mm.
this took a little while to type and surely you could have just googled it or thought about it for 3 to 7 minutes and came up with a solution