There are discussions in various threads. Even if your spacer is correct, some say the wheel could possibly be machined too wide. If that is correct, the only safe way if you're really worried is to measure your own wheel. It's not that hard when you've pulled the bearings. The spacer should be a little longer than the width between the edges in the wheel for the outer races, like one or two tenths of millimeters. I'm not at my best writing in english here but I guess you understand the reasoning: the outer races must not be at tension, just the inner races.
This bloody great post by Pete explains it in better english, in rear wheel context. The whole thread is hilarious.