Your amount and VAT choice stay only on this page. Reloading or leaving clears them.
How the VAT is worked out
To add VAT, multiply the amount before VAT by 15%, then add the tax to that amount.
To remove VAT from an inclusive amount, multiply the total by 15 and divide by 115. Subtract that tax from the total.
VAT is rounded once to the nearest cent, with half a cent rounded up. Separately rounded invoice lines can give a different total.