Billings Pro INVOICE template CUSTOMISATION

I have a basic customised Invoice template I made up in Billings Pro, but I need a field that shows a deposit amount required at the top under the total, for them to secure booking as well as subtracting the balance on the Total below. I do not necessarily need you to make up the exact template but at least give me the code needed and talk me through where to put it is fine.. In the sample below is part of my template that I did up.. the features I really need down is in RED but obviously showing the correct balance at the bottom.