Monitoring Disk Utilization

Hi. I'm trying to setup an alert for disk utilization. However, when creating the threshold, I just realized that it can only put threshold by percentage, and not by direct numbers (in MB or GB). It is quite difficult to put threshold by percentage, since it is more natural to put direct numbers there (critical if below 100 MB, regardless of total disk size, for example). Is it possible to put numbers instead of percentage? Maybe by using a different monitor?

Also, if I have to use percentage, why I can not put lower than "1"? For example, I can not put "0.1". If the minimum is 1 percent, for large harddrive (eg. 120 GB), 1 percent is equal to 1.2 GB, which is too large to put as an alert.

Thanks.

- irving