Goal Metrics - Using a Rollup Field

Hi

I am trying to setup two goals for our sales team one of which works fine but the second one I'm having a problem with.

I have a custom entity which is linked to an opportunity record which has an option set with a number of items in the list such as fees, services, training etc.  When they are selected a price value is also specified in the record.

On the opportunity record I have a calculated rollup field to add up all of the fees and services and provide a total value "One Off Fees".  The sales team are have a target on their opportunities off these fees and services and I want to create a goal for them to monitor this which I've created against the rollup field but no data pulls through against the goal.  Do goals now calculate against a rollup field?

Can anyone advise if this is possible or a workaround?

Thanks

Robin