So in invoices we have fields for:
VATable
VAT Amount
Total

Now I want to know where can I see the calculations for the VATs until to the total. I have already viewed each of the fields but I don't see any calculations. 

I wanted to review the calculations and process behind it because instead of subtracting the taxes on total sales , it adds all the taxes to the total sales.

I hope someone could help me out thanks!