Setting default image width after uses selects an image

I'm using a custom filebrowser for adding images into the CKeditor. When a user selects an image I already fill in the 'alt' automatically which works fine. However if I try to fill the width box with a default value of 250 it seems to be overwritten.
I believe the txtUrl onChange function overwrites my value.

This won't work due to stretching (plugins/image/dialogs/image.js):

original.setAttribute( 'width', '250');

I can changing the inputfield with

element = dialog.getContentElement( 'info', 'txtWidth' );
element.setValue( '250'); 

I even tried to calculate the height myself in changing it in the dialog accordingly

elementWidth = dialog.getContentElement( 'info', 'txtWidth' );
var width = elementWidth.getValue();
elementHeight = dialog.getContentElement( 'info', 'txtHeight' );
var height = elementHeight.getValue();
var newHeight = Math.round(height / width * 250);
if ( !isNaN( newHeight ) ) {
  elementHeight.setValue(Math.round(newHeight));
  elementWidth.setValue('250');
}

It can kinda work when I do this:

function resize(dialog) {
  var width = dialog.getValueOf( 'info', 'txtWidth' )
  var height = dialog.getValueOf( 'info', 'txtHeight' )
  var newHeight = Math.round(height / width * 250);
  elementHeight = dialog.getContentElement( 'info', 'txtHeight' );
  elementHeight.setValue(Math.round(newHeight));
  elementWidth = dialog.getContentElement( 'info', 'txtWidth' );
  elementWidth.setValue(250);
  updatePreview( dialog );    
}
setTimeout(function(){resize(dialog);},300);

However this isn't a good solution due to the setTimeout usage.

Anyone have some suggestions on how to properly solve this?