I added the following custom field with the formula: [Remaining Work] / [Remaining Duration] Now I want to add an "if" condition, meaning if a task is 100% complete, that no value or the value "n/a" is given. I found the IIf function is the correct one, but I only get errors with the different ways I have tried to write the formula, e.g.: IIf ([% Complete] = 100, "n/a", [Remaining Work] / [Remaining Duration]) or IIf ([% Complete] <100, ([Remaining Work] / [Remaining Duration]), "n/a")