Calculated Values

I'm working on my first real Cocoa application, and I'm stuck. I'm using Core Data and want to show a calculated value in the user interface. I have a master entity that has a one-to-many relationship with a subordinate entity, which has two two number properties. I want to subtract the sum of number2 from the sum of number1 for the related subordinate entities and show the result in a table cell.

I can't figure out how to do this. I've done a fair amount of work with WebObjects/Enterprise Objects, and WO/EO would allow me to define a method to do this calculation and then bind to it from the user interface.  Core Data doesn't seem to work this way. I'm sure there's a "Cocoa way" to do this, but I haven't found any docs or sample code that are on point.