tax code

i create one tax code 4% this tax code assign code one ledger account when i post business transactio ns in f-02 amount value 20000, but actually tax amount value 800 total value 20800 but system showing total value 20000 tax value around 720 base value 19280 its including tax but i need excluding tax like 20000 base amount 800 tax amount and 20800 total value i need this one