Formula to generate the remaining amount without tax for Partially paid transaction types in proportion of amount paid to total amount

For open invoices partially paid, consider the following example…  

Subtotal /{ amt (net of tax) } =370.00 –> This amt is without tax

Discount Item= 0

Tax Total =55.50

Total /{ total gross amount } = 425.50

Amount Due / { amt remaining }= 287.50

Amount paid= 138.00

 Since we need to get the amount remaining without tax , it can be considered in proportion as  

 

 

Hence, 287.50 / 425.50 = amount remaining without tax / 370.00 

 

Amt remaining without tax= 287.50*370/425.50 = 250.00 

 

                      = amt remaining * Amt net of tax / total gross amt