How do I calculate the total wattage of a dc to dc circuit?
No.  Those are the correct units, but the answer will be off.  This problem can be solved with simple physics plus common sense; specifically the conservation of energy plus the fact that no power conversion device is 100% efficient.

You need to calculate the power available from each output, by multiplying its current by its voltage (e.g. each one of your two 3.3V supplies needs 1/2 A, and so provides 1.75W).  Then you need to add all of those together.

The number you get is the maximum possible output power.  It's also the theoretical minimum of the maximum input power.  In reality you'll need to divide that by the efficiency of the supply (e.g. 80%).  If you really want to get down to details, each output will have its own efficiency, so you'd need to multiply each output power by the individual efficiency, then add those up.