Slack Calculation in Summary Task - Project 2003

I am trying to show the minimum slack value for a summary task on my project. It seems only to show Maximum, which seems to be not very helpful in managing a project.

Is there a switch or option burried somewhere to set this to minimum?

At the moment I have set a Customized Duration field to equal the Finish Slack and set the rollup to minimum.

Thanks for all the help...