When you are using this function and you change a value to be able to recalculate the items you have set to equal so they can be easily calculated do not recalculate. You have to uncheck a box then recheck it then recalculate as the calculate box is greyed out until you change one of the checkboxes.

Ideally, this would function would recalculate on the fly as you change one value or as another solution just never grey out the calculate button so after you make a change you can just hit the calculate button without unticking and re-ticking to bring up the calculate button.