How do I enter the calculation F! xF2/C into the custom calculation script box of a text form field

Is F1, F2, and C are the field names involved, and you want the field to be blank when the denominator evaluates to zero, you could use something like:

// Custom calculation script

(function () {

    // Get the field values, as numbers

    var F1 = +getField("F1").value;

    var F2 = +getField("F2").value;

    var C = +getField("C").value;

    // Set this field's value

    if (C !== 0) {

        event.value = F1 * F2 / C;

    } else {

        event.value = "";

    }

})();

If you want to round the result, to the nearest one hundredth for example, that one line would be:

        event.value = util.printf("%.2f", F1 * F2 / C);