Increase shipping field by x% with a button

I’m trying to add a button to Sales Order that would increase the already calculated shipping rate (so let’s assume somebody already got the rate returned by UPS for exemple) by x% and set the value to the newly calculated rate.

I was able to script a simple button and read a message sent to a client script.

I don’t get how you do you listen for new modification in the record once the script is loaded. I can get the field that are loaded at the init, but not sure how to get the entry made by the client.

It should be pretty simple. Listen to the event, get the value of the field, apply x% and set the new value to the field. but hum no luck so far. Any help would be very appreciated.

Thanks