Set total from Details page on Master page field and save to db

Hello,

I have an Add page with a details table. I am trying to get the total from the Details page, set the value of a field on my master page to the details total and then save it to the DB.

I am following the instructions from this article https://xlinesoft.com/phprunner/docs/show_order_total_on_the_edit_page_as_the_details_table_is_updated.htm which works fine on setting the value but it is not saved on the DB. The master field is showing as read only and although it looks like it is getting the value from the total , it is not actually saved on the DB.

Details Page

Master Page:

Any help is welcome

Best
George