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I am working with the invoice source code from
http://www.latextemplates.com/template/invoice

I want to display the "Webhosting" value as a percentage of the Subtotal.

In the example pdf,
Subtotal = 734.55
Webhosting = (734.55 * 3.9 / 100) + 0.3 = 28.95

If the Subtotal is 734.55 then ...