Check Excel Formula parts with F9
When formulas get complex, it can be useful to check each part to ensure it's producing the result you're expecting. One way to do this is by using the F9 key (it's the same on both Mac and PC keyboards).
By highlighting part of a formula and pressing F9, the highlighted part will be replaced with the calculated result. When you're done checking the parts you're interested in, you can just hit "undo" to revert the formula back to its original form.