How do I calculate my profit margin percentage?

If I have a contractor that I pay at $500 a day and I am charging a client for that contractor at $800 a day but I have to also pay 9% of cost (for insurance) how do I calculate my pure profit percentage and how can I apply that percentage to other rates?How do I calculate my profit margin percentage?
Margin percentages can be calculated on any net margin line in your Profit and Loss. Margin is usually a calculation on the funds made ie the $800 and not the cost ($500). Divide any of your Net Margin lines into the $800. ie $300 margin into $800 =37.5%. Or an additional margin line after on-costs (the 9% or $45) could be performed - $300 less $45=$255 divided into $800 equals 31.8% Margin.