Add the percentage sign to your formula in the first post for the rich text field. Put it directly after where you are placing $margin.
You might be able to just add a new variable up top for this as well. This may or may not work as I typed it up. This is just a quick example. It still uses marginOne for the calculation(as that is a number) and then inserts marginTwo for the text. If you didn;t want to do multiple variables here you could also just put "%" in the formula below like I first mentioned. I just found this easier.
var number marginOne = [Total Job Gross Margin F];
var text marginTwo = $marginOne + "%";
If( $marginOne >= 0 , "<span style=\"background-color:#fbff00; font-size:16px; font-weight:bold\">"&$marginTwo&"</span>", "<span style=\"background-color:#ff0000; font-weight:bold\">"&$marginTwo&"</span>" )