Multiple Check Boxes Calculation to get % Complete

I have five check boxes that represent different phases of a construction project. Each check box is worth some of a total number of hours to complete the project. I want to create a field that calculates the % complete of the project based on which check boxes are checked. For example, if all five check boxes are worth 20% of the hours, if one is checked I want to indicate that we're 20% complete. If three are checked I want the field to show we're 60% complete. I know how to do this with five extra fields but assume there has to be a way to do the full calculation in one big formula field.