I know this is not what you are after but I'm not sure what it is as the way you ask don't add up. So hear is as close as I can get and some electricity 101 to help you out.
Everything in the truck is 12 volt, so everything runs on 11 to 14 volts depending on how much is available to be had in the hole system at the time. All the volts do is move the amps to the work and the total amount of work going on is watts, volts X amps = watts.
So the amps that do the work and glow plugs pull a lot of amps, about 8 amps per glow plug or 100 watts worth of 12 volt power. Thats about 60 amps for all 8 of them or 800 watts of total power draw when on.
The only way to know what the real world draw is would be to install an amp meeter in line between the battery and gpr. But the only reason to do that would be engineering work.
For repair work testing all you need is a test light. By hooking the test light to the + side of the battery you can probe the glow plugs one at a time, just unplug the wire from the GP and probe it with the light. If the light comes on full the GP is good. I am not sure on this but I think you can probe them one at a time at the valve cover gasket plug, if I'm wrong on this someone will set me right I hope. I will check that on my truck tomarow any way, I like to know for sure.
Now I hope that helps and if not I'm glad to confuse you more.
My baby is a one of a kind.
1977 I bought a new 76 F150 club cab.
Sitting on a 84 F350 6.9 4X4 chassis
With all the drive train and goodies from a 95 4X4 4door PSD.
Nothing stock left on her and not a nut, bolt, hose, or wire on there that I didn't put where it is.
The Wifes Ride:
2002 Excursion Limited 7.3 PSD
Themo King/Mikuni coolant heaters on both to keep things toasty without idling.