Try something like:
// Custom calculation script
(function () {
// Get the field values, as numbers
var numerator = +getField("H25").value;
var denominator = +getField("H22").value;
// Perform the calculation
if (denominator !== 0) {
event.value = numerator / denominator;
} else {
// Blank this field if denominator is zero
event.value = "";
}
})();
Replace H25 and H22 in the code above with the actual field names you're using.