I have a compute field let's say, (Special Calc) that does some beforehand calculation on the move line and saves it in a field. 

Now, say for example, On entering a tax (special tax) in taxes_id in invoice line, the special tax amount should get updated by a certain percentage of the amount in special calc field which at the end will update on tax_totals field of account.move in odoo. 

 By far, all I was able to change the tax value from the front end from here, updating Untaxed Amount in the groups_by_subtotal​ dictionary.
Now I am stuck and not able to find a way, which can lead me to the solution. Any help, including any references will work as well.