I don't know how much do I need to charge for a deposit or If don't have to ask for a deposit.
The cake is going to be $550.
I do exactly the same - half to confirm the booking and the remainder due 2 weeks prior to the event. I say no changes a month before the event - maybe I'm just mean (or a control freak?!)
I charge a $30.00 deposit. The balance is due 2 weeks prior to the wedding (no changes are accepted after balance is paid).
The deposit is refunded to them after they return my supplies from their wedding cake.
I find that I like to get paid as late as possible, while still being smart, because it's added incentive for my hard work. I hate it when I spent the money 3 months before I even started the cake!! 
haha
Lindsay
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