How I can update a custom field parent task using the value from a subtask custom field?

[Juanjo Pérez]

How I can update a custom field parent task using the value from a subtask custom field?

Example: I have a task with custom field "Total holiday days", and a subtask from this with another custom field "Holidays demanded". Then I want that when the subtask custom field "Holidays demanded" is updated then the task "Total holiday days" is update with a substract.

On the example where is the "1" I have to update the "customfield_10029" using a substract from the subtask customfield.