Bob, as Jon B. wrote, you need .25mm shims so that you can make minimal changes and sneak up on the spec. Be sure that you don't accidentally change the total shim thickness when you start changing shim sizes! Also, when you get it close remember to make a measurement in 3 or 4 places.
I assume that your 3.3 shim is in "S1" (ring gear side), and your 3.1 shim in "S2," correct? That means no matter what you do you want to end up with a total thickness of 6.4mm. Because your allowed tolerance is .12mm to .18mm, you don't have far to go. You want to reduce the thickness of S2 in order to allow the ring gear to get closer to the pinion head (reduce back lash).
If you go from 3.1 to 3.05 (S2) you will need a 2.8 shim and an .25 shim. To maintain your 6.4 total you will need to go up to 3.35 on S1; a 3.1 shim and a .25 shim. 3.05 + 3.35 = 6.4.
If you have 3.3 in S2, and 3.1 in S1 (doubtful), you have to make some different calculations, maybe start by flipping the two shims to see where you are. Just be extremely careful to not lose all of your back lash while tightening the side cover; you don't want to damage the gears. Also, always be sure that the bevel on your shims faces toward the diff.
You might want to look through this thread...
http://forums.pelicanparts.com/porsche-911-technical-forum/469037-915-rebuild-part-ii-time-assembly.html?highlight=differential+back+lash+915