I dont have the answer off the top of my head, but you can check yourself.
Get the 172 cup track, and your track, minus yours from the 172 cup track and yo have your difference.
Now the offset on the cup wheels is 38 as opposed to 48 on the 172. SO thats an extra 20mm simply from offset. Compare your offset to that of the cup and you should be able to work out the difference needed if the cup offset isnt enough.
Word of warning though, simply spacing the wheels out will affect steering geometry and make your car handle strange and poorly.