Instead of a calculation field, you might want ot set up a summary report that groups your records by category and computes a sub-total for each. If you haven't set up summary reports before, you might want ot look at this link: Here's a link to a simple tutorial on setting up summary reports that you may find useful:
Creating Filemaker Pro summary reports--Tutorial
That approach may or may not be applicable depending on how you've set up your database.
How many tables do you have and how are they related? Your example of sum ( budget::Estimate ) suggests you might have two tables linked by a relationship but I would need to know more before I can suggest anything more.