Not sure how you came up with all of those numbers.
You should be measuring how many Amps each devices uses and at what voltage.
Most devices will list their power requirements. Harddrives, cdroms, dvd's, etc should show on the label.
Lets say I have two components:
my DVD drive requires:
My laptop harddrive requires:
Now add it up. Let's start with 12v:
Dvd(0.8A) + Harddrive (0A) = 0.8A
Dvd(1A) + Harddrive (1A) = 2A.
So the power supply must supply at least:
The wattage is more of a generalization on power supplies since it lumps all of the different voltages together.
to find Watts, you multiple Current(Amps) times Voltage.
so 12v * .8A = 9.6W
5v * 2A = 10A
for a total power consumption of 19.6W
If the device doesn't list it, then you can just measure it with an Ammeter. Most meters will measure Amperage up to 10A.
You simply put the meter in-line with the device. Power up the computer and watch the needle to find the peak.