Deriving a Non-Formula Field Using a Before Trigger

The simplest way to derive a stored field's value is to create an appropriate "before" trigger (Before Insert in Database and/or Before Update in Database) which assigns the field's value to your calculated value.

See Testing Whether a Field's Value Is Changed for more information on this function and Avoiding Validation Threshold Errors By Conditionally Assigning Values for a tip about how to avoid your field assignments from causing an object to hit its validation threshold.