Crystal Reports Sum Allowed in Chart but not as Formula

Message

   Just as the Subject title says. I'm creating a report and I want to sumarize a number field that is based of the formula {Break}: 

if {ReasonCode}='Break'   
then cdbl({TotalMinutes})
        else 0

{TotalMinutes} is a formula that is:

if {LogIn}>{LogOut}
then datediff('n', {LogOut}, {LogIn})
         else 0

{LogOut} is a DateTime field and {LogIn} is a formula based on the same DateTime field:

if next{ResourceName}={ResourceName}
and date(next({DateTimeField}))=date({DateTimeField})
then next({DateTimeField})
else date(0,0,0)

I can pull {Break} into a chart as Show Value with the Summary Operation of SUM and it displays exactly what I want. But Crystal will not allow me to summarize the formula field in a formula, using the SUM function or as a Running Total.

Can someone explain to me why?

Thanks.