It's alot of trial and error, I went thru this same thing a few weeks ago and am now about to pull the motor to replace the flexplate and take care of a crack in the block, if your starter drive is to close to the ring gear use a double .020 shim and if that doesnt do it try and .040 , if its still not right use both.
If the starter is to far away from the ring gear use single shims on the outside bolt using the same sequence.
If all you can find are double shims you can cut them down the middle to use as singles.
Also found this very interesting to anyone having starter problems.
http://www.egrproducts.com/html/tech_tip_8.html