Custom Calculation Help

It could be something like:

// Get the field values, as numbers

var D13 = +getField("D13").value;

var D14 = +getField("D14").value;

var D15 = +getField("D15").value;

var C13 = +getField("C13").value;

// Perform the calculation and set this value

event.value = (D13 + D14 + D15) / 15 * C13

Replace "D13" etc. in the statements that use getField with the actual field names.

If using the simplified field notation option, it would be:

(D13 + D14 + D15) / 15 * C13

but again you'd use the actual field names. If the field names have spaces, punctuation, or certain other non-alphanumeric characters, you have to escape each such character with a backslash when using this option.