SpamSieve one year later

I've now been using SpamSieve for just over one year. I'm fairly happy with the performance of it:

Filtered Mail
123922 Good Messages
15275 Spam Messages (11%)
40 Spam Messages Per Day

SpamSieve Accuracy
184 False Positives
357 False Negatives (66%)
99.6% Correct

2007 Good Messages
2387 Spam Messages (54%)
143044 Total Words

519 Blocklist Rules
2941 Whitelist Rules

The false positive rate is still a little high for my liking: about one legitimate message every two days falsely marked as spam. This is one of the reasons why I'll never turn on automatic deletion of messages flagged as spam. When it comes to this type of error, how much is too much? One message per day? Per week? Per month? Per year? For me even one lost message per month is too much for my liking, but I think I could stomach one per year fairly happily.