So in the HeroLab builder Street Cred is calculated using the Karma/10 formula. And it can't be reduced below that, and one of the people on the thread listed below is saying that there isn't any absolutely lower limit that Street Cred can reduced to when buying off Notoriety. And the programer needs to know how to handle it

The section on Street Cred, Notoriety and Karma usage isn't terribly clear on this.

http://forums.wolflair.com/showthread.php?p=62059#post62059

There is a lower limit. 0

From SR4A, p265 -
"Characters can reduce their Notoriety by permanently sacrificing some of their Street Cred."
"Street Cred is based on a character's total earned Karma..."
"...additional points may be added to a character's Street Cred..."

Street Cred is earned, just like Karma (as well as alongside Karma). You cannot sacrifice what you have not earned, so it cannot go below 0.

That stipulated, if Hero Lab does not allow sacrificing Street Cred below the Total Karma/10 calculation, then it has a bug that should be fixed.

At the beginning of the day, you can always be a nobody.

Of course, since karma awards were increased in 4A, the basic street cred increase rules have you increasing your street cred ever 1.5 runs.