Unfortunately this is no magic number that I can give you. Generally we will tell you to lower the score a few points then monitor for false positives. After a few days if you don't see any lower the score again. You can do this untill you start to see a few false positives then adjust the score until you are catching the maximum amount of suspected Spam and getting the fewest number of false positives. Every domain is different and what you may consider Spam may not be to another domain.
As for the influx of Spam. Please see our state of Spam report (link below) for info.
http://www.symantec.com/business/theme.jsp?themeid=state_of_spam