Made this a while back...
Before we start, please use Roan's (zeroluxxx) DIY Guide on measuring wheel parts so you get the correct calculation to start out with: [ http://forums.vwvortex.com/zerothread?id=3773369
Maybe this could help some people as I know it can get confusing. Let's start with my wheels as an example:
17x8 ET43 (7" barrel + .5" center + .5" lip)
17x9 ET53 (7.5" barrel + .5" center + 1" lip)
Adding the center's thickness into the equation only works if your wheels are sandwich mount (lip, face, barrel).
I moved the 1" drop lips from the rear to the front, resulting in:
17x8.5 ET37 (7" barrel + .5" center + 1" lip)
Now how did I get the final offset? Using [ http://www.1010tires.com/WheelOffsetCalculator.asp
]. I input the stock set up (8 ET43) and input the new width (8.5) and got to ET37 to maintain the same inner position because you aren't changing anything except the outer position.
I have noticed for every .5" you go out (pattern), while maintaining the same inner position/clearance, you will lower your offset by ~6mm (6.35mm to be exact (25.4 / 4 = 6.35). I have also noticed the outer position will go out 12mm (using the calculator, it gives you 12mm) (12.7 to be exact; 25.4 / 2 = 12.7)).
Then I got 1.5" lips to replace on the rears resulting in:
17x9.5 ET47 (7.5" barrel + .5" center + 1.5" lip)
Now what about if I changed the barrels up? You'd see the same pattern, but the outer position stays the same, whilst the inner position goes in or out (depending if you get smaller or larger barrels, of course).
Let's use my stock fronts for example (17x8 ET43 with a 7" barrel):
I'll put a 6.5" barrel on it, resulting in: 17x7.5 ET37 (6.5" barrel + .5" center + .5" lip). Notice the pattern again; .5" on that inner position results in 6mm lower offset (again, technically 6.35mm, easier to round down). Same thing with the inner position, it would gain 12mm (12.7 to be exact).
Well, what if I do both [smaller barrels and bigger lips]? You basically use the same formula/input the same things, but one at a time.
For example, lets use the stock fronts again but put .5" smaller barrels and a 1.5" lip on it.
Stock we know is 17x8 ET43 (7" barrel, .5" center + .5" = 8")
.5" smaller barrels we know results in 17x7.5 ET37 (6.5" + .5" + .5" = 7.5)
Now lets add that 1.5" lip to the equation: 6.5 + .5 + 1.5 = 8.5"
Final specs would be: 17x8.5 ET24 (which would gain 12mm inner clearance and outer position would "poke" 26mm more than stock lips).
Hope this helps