Before You Begin: You need knowledge of logical operators and the SQL programming language to create custom totals.
In both column totals and summary totals, you can create a custom total that is based on an expression that you build. When you’re creating a total, select Custom in the Function drop-down list, then Edit Custom Function. The Custom Field Editor opens, and you can build your expression to create a custom total. See Custom Fields.
Note: When you create a custom field for a total, you must include at least one math function in the expression (sum, count, max, min, or average).