Let's say there are three fields on the Add/Edit page: Price, Quantity and Total. To calculate the total value on the fly, use the following JavaScript code (add it to the Add page: JavaScript OnLoad event or Edit page: JavaScript OnLoad event on the Events tab).
var ctrlPrice = Runner.getControl(pageid, 'Price');
var ctrlQuantity = Runner.getControl(pageid, 'Quantity');
var ctrlTotal = Runner.getControl(pageid, 'Total');
function func() {
ctrlTotal.setValue(Number(ctrlPrice.getValue()) * Number(ctrlQuantity.getValue()));
};
ctrlPrice.on('keyup', func);
ctrlQuantity.on('keyup', func);
See also:
•JavaScript API: Control object > getControl()
•JavaScript API: Control object > setValue()
•JavaScript API: Control object > getValue()
•JavaScript API: Control object