A family of Microsoft relational database management systems designed for ease of use.
I just stumbled across a post in another forum with the same apparent issue.
Like the other poster does it work with integer values but doesn't work with fractional values?
I just tried to reproduce the error using fractional values and in the Total Slack Field - I get the same issue. Other fields appear to work correctly (Duration etc).
However, when I create a new file, I don't see this issue. Was your file created in Project 2007?