...
so if you have an FP% of 0.1%03%, and an FN% of 2.5%47%, the batting average is
| No Format |
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(1 - (2.547 / 100)) "/" (0.103 / 100) = .9759753/.001 0003 |
That's actually the correct batting average for SpamAssassin 3.0.0's scoreset 3, measured against the validation corpus when we released it. 
See also MeasuringAccuracy for other schemes used, or FpFnPercentages for the main one we use in SpamAssassin.