If you set the export value of the checkbox to 5, the script custom calculation script can be:
// Custom calculation script
(function () {
// Get the field values
var v1 = getField("SKL_1").value;
var v2 = getField("TRN").value;
// Perform the calculation
if (v2 !== "Off") {
event.value = v1 * v2;
} else {
event.value = "";
}
})();
You didn't say what you want the result to be if the checkbox is unchecked, so the code sets it to an empty string. If you don't want to set the export value of the checkbox to 5 for some reason, that one line could be changed to:
event.value = v1 * 5;