I'm working with mostly all custom fields in my project, such as manually entered Start dates and Finish dates. I'm not sure why I cant use a "duration1" custom formula to calculate my projected and actual duration.

My formula looks like this [Start4]-[Start3]

Start4 is my "projected start"
Start3 is my "projected finish"

I even tried doing it as ([Start4]-[Start3])/480 so it used minutes per day, but that didn't work either.

please help.
thanks.