I've heard the actual thickness of aftermarket shims is less than precise. Several guys have used a similar appliction to select shims and ended up with clearances that weren't where they should be. With the aid of a micrometer it's been found some shims aren't exactly the right thickness. I remove the old ones, bag 'em up, mark the bags with the position and measurement, then take them to the local shop and we measure with a micrometer to select replacement shims. Last time I did one we found a replacement shim that was a thousandth under what it was marked.
For folks who don't have the option of driving to the shop to get 'em, the calculator will give you theoretically correct shim. Whether or not the sweat shop worker in China made that shim to exact tolerances seems to be a crap-shoot any more, but hey, what choice do we have?