Hi,
Yes be very careful when measuring the line voltages because these are definitely deadly, and being in the basement it's easy to have the feet grounded and accidentally touch something and this is how the heart gets stopped in many electrical shock accidents (arms through body through legs through feet to floor or ground).
You also need a meter that is definitely made to measure these kinds of voltages, keeping in mind you might encounter 440vac if it happens to have that in there also.
There's the "One Hand Behind Your Back Rule" too, for measuring voltages...this is where you connect the black lead to say neutral with a clip lead all with one hand behind your back and then take the red lead and probe with that one hand only. This prevents accidental hand to arm to body to arm to hand electrocution.
I would think if you measure 120vac then you have 120vac, if you measure 208vac then you have 208vac, if you measure 240vac then you have that, etc., using a meter set to read ac volts. There are a number of ways to extract power too, either line to line or line to neutral (Y system), so you'll get a different reading line to line than you will line to neutral.
If you know someone who works with this stuff you could ask them to take a look too.