I think we might have solved it.
If you go to the power supplier's spec sheet you might find a listing of the open circuit voltage.
I had an instructor that used to say "It's all Ohm's law". That did not make his class any easier, but it stuck with me.
Once the current starts flowing, there will be a voltage drop due to resistance in the welding circuit.
You will find the most accurate reading by measuring directly across the welding arc, but this might be hard to do with a hand meter without being blinded.