Since SARS has set the VAT rate in South Africa to 15%, the calculator uses the decimal representation of 15% (0.15) to automatically add and subtract the VAT portion for any Rand Amount.
Because VAT is charged at 15%, it means that to find the VAT amount to include in a pre-VAT Rand value, you need to multiply the Rand value by 0.15%. The answer to this calculation is the Rand value of the VAT portion, which then needs to be added to the original amount to get the VAT inclusive amount. The VAT inclusive amount will always be more than the amount which excludes VAT.
If an amount already has VAT included, you can find the VAT excluded amount by dividing the original amount by 1 + VAT percentage (which is 15% in South Africa). In other words you can find the amount which excludes VAT by dividing the amount that includes VAT by 1.15. The Rand value of the VAT amount is then just equal to the original amount less the amount which excludes VAT. The amount excluding VAT will always be less than the VAT inclusive amount.
For more information on the Maths behind the calculator, check this article on how VAT calculations work