The only way to calculate that value is through the GEPF Calculator, but there is a problem. The website which offers this calculator for users is hosted on a secure site. The SSL is not working on that site. SSL ensures that your data is received by the server and no one else is snooping through the connection. This means the data is safe over the Internet. If you try accessing the calculator, an error will be presented.
This could also mean that the website is accessible only for residents of the country where the calculator is eligible. Visit the website below and click on calculator. To get additional info, contact them using the email on the following page: http://www.gepf.gov.za/index.php/contact_us
To find more information access the website: http://www.gepf.gov.za/