if then formula

I don't have much experience with using the form function in acrobat and need some help with a calculation. It could be pretty easy for someone who knows what they are doing!

The pdf form I am creating will allow a customer to type in the quantity of a product and then see how much it will cost them. The spreadsheet will span 3 pages. Each page will have a total at the bottom with the grand total on page 3. If this grand total amount ($) is $500 or more than 10% will be taken off the grand total.

I cannot figure out how to get the 10% off ONLY if the grand total gets to be >=500.

any thoughts?

Thank you,
Anna