How do I get the custom field to evaluate on existing records when I add or modify a Formula?

The formula simply returns the value 1 so that I can access it as a summable field from some custom reports I'm running.

Matrix Report:
Rows = Potential's Owner
Columns=Territory (Custom Field)
Subtotal On=SUM(My Formula) ~ Sum(COUNT OF POTENTIALS)

Is there another way of running this report
Thanks