Just display a certain (set) percentage of one field to another field

So I have a form where I have 2 fields I need to display a percentage of. So I need to display 50% of the amount entered in a field named "WebDesignAmount" (field is formatted as a number with correct decimal places etc) in the field named "Deposit" (formatted the same as the other field). I assume I would need to create a hidden field to determine the 50% (or maybe not) but I don't know how to do the calculation. Could one of you amazing Acrobat geniuses help me with this? I'd really, REALLY appreciate it! TIA!!!