Yea, this is kind of a known issue, I probably should have just removed that field for time being. The program reads simplewallet output while syncing to find your transaction history, but I think it occasionally misses some, thus the negative balance, or i goofed the math somewhere. I'm thinking to just parse the logfile rather than watch output, as that way it should be impossible to miss anything.

The locked and unlocked balances are the ones to go by, and if they're green, that means you should be synced and ready to send transactions.