So, I just completed the first full week of keeping statistics for SpamAssassin.
I had the following results for my personal inbox:
A False Positive is when Spamassassin identifies a message as spam and it isn't. I'm rather encouraged by the 0 there.
A False Negative is when Spamassassin identifies a message as clean when it is actually spam. In all but one of the cases, I was able to come up with something identifying it as spam enough and the rest of my mail as not so. The one case I couldn't get was too short and too like much other mail that I get to be able to filter it without having false positives.
This does mean that 78.5% of the mail that I get destined for my personal mailbox is spam and only 1 out every 5.6 messages is non-spam. Kind've scary.
The next goal is to apply it to my entire received mail rather than just my inbox.