When you click on the "..." button next to the custom field a window appears:
The Caption field lets you give the custom field a name that is meaningful to you and is shown to the left of the field in the calculator.
The expression field determines what is shown in the custom field. You should use the replacement marker %x% in your expression to represent the calculator's results. For example, if you want your custom display to always multiply the calculator result by two you would use an expression as follows:
%x% * 2
If you used this example expression the custom field would always show a value twice that of the Usual result:
The custom expression can be any macro-compilable expression.