You can apply a foundation for a grant as outlined in this example. When the application has been reviewed it might be approved and some funds might be assigned to the applicant.
Then the user needs to split the amount into several lines and would like to know the remaining value to fill out:
Rest amount is a computational field which is calculated as: Sum – Line1 – Line2 – Line3
A response arrow from each field ensures that Rest amount is executed, and thus re-calculated, whenever a value is changed.
Opening the Activity Editor and selecting form in the model you can test the behavior.