This question doesn't really have an answer. Let's say I have a Valab attenuator and a source that puts out 5V. I'll probably only use maybe the first couple of steps.
Maybe you have USB powered DAC hanging off your computer, or you're streaming music from your cell phone. Either one will probably be ~2V, meaning you'll need to crank the control up a bit.
This also assumes that the two instances are using the same pair of headphones, which will also influence where your level control is set.