Yes, something like this can be done.
You have 2 choices. The first is the easiest. Us an acf/load_value filter on your place holder field. Get the values from the other 2 fields, if they exists, an do the calculation and return the result.
The second would be to create custom JavaScript to populate the field without doing a page load. You’d need to do some more research on this. You can start here: http://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/
