Adobe LiveCycle Designer ES8.2 - Patterns




I am making a PDF form and need some help with patterns.

The form I am trying to create has a box for a "program fee" where the user input the fee cost the next box is the "x number of attendees" where the user puts how many people at attending etc.. I then have a box "subtotal" which multiply the "program fee" with the "x number of attendees". What I need is to have a box where I can add the pattern to work out the GST of the "subtotal" GST is 10% of total cost.

Does anyone know the pattern to work out the percentage of another box i.e the "subtotal".

Once I can get the "GST" worked out I want to be able to have a "total" box where the "subtotal" and "GST" add together.