You can add unbound value boxes to the reports and program them to do any math that you want(Except divide by Zero as that is impossible) If you add an unbound value box named D to the Group Body of your report, and want to perform the equation that you listed, the programming would simply be

D = (A-B) * C

You will want to format the D element as a Number or Money value, which will not only apply formatting to it but allow that value in the group body to be Summed by calculation boxes in the group footer.

If you are trying to do math on the result of Summary Calculations that becomes a bit more difficult as the Summary Calculations happen AFTER programming happens so that they can sum calculated values. In this case what you do is have duplicate Summary Calculations elements in a group footer directly BEFORE the one you are working in. Set them to be invisible. Then in the one you are working in, add an unbound value box, format it as the appropriate numerical type, and program it to perform the calculation using the element names of the Summary Calculations elements in the group footer above.

-Ray

It isn't an issue of efficiency. Its a matter of flexibility. Because you cannot set the formula, like you can in programming, they are less flexible. They end up being a lot more efficient, in that they are run in compiled C, rather than interpreted SBasic.

So, if your formula is simple enough (sum, average, etc.) a summary calc is going to run faster and use far fewer resources. But if you need a custom formula, you can implement anything you might need in SBasic. They can also work together in combination, as Ray made clear - with some caveats.