Custom field and Total field auto calculate value in Grand Total


I have a custom field that I added in the grand total section of my Quote Worksheet.

I would like to:

1. Auto add it to the Total value

2. Also in relation to the custom field I created, the value was not updated based on my calculated formula until I saved the record.

Is there any way that the value of this custom field will be automatically updated after a dropdown selection trigger event occurs?