make the application fixed

Dear Sir

please i need your help

in purchasing orders i need 2 things

1st thing is to edit the files to allow the purchaseing taxes to be excuded from the total amount like

i purchased some thing with total amount 300 USD and it require 0.5% Tax so it should be 300 - 1.5 = 298.5 but the program adding them to be 301.5 and its wrong

2nd i need to apply that purchaseing tax only to the orders which are over than 300 USD like i need to use this code

if (total amount >= 300)
{
   apply the tax
}
else
{
  don`t apply
}

how can i make them and which files should i edit

thank you .....