Do not use a DMM and voltage + that useless formula to set anything..
An amp's gains adjust where the amp reaches it's full power on the head unit's volume scale. If the head unit clips @ 40 don't turn it above 38? Set your amp to reach it's full power @ 38 using the gain knob.
To do this you will need an oscilloscope. Turn the radio to volume 40 (forbidden zone) and run a tone. Put the scope on the amp and adjust the gain until the amp just starts to clip (At volume 40) Then don't turn it up past 40 because at 40 it clips at that tone.
Now repeat that on every frequency to avoid clipping on any frequency.
That formula is total garbage because a sub can ricochet back and forth from .5 ohm to 100 ohms in seconds..And a DMM doesn't show anything but voltage..
To use a *true RMS DMM* and that formula to test an amp's wattage you'd need to attach a "x ohm" resistor to the amp and run a series of tones. A resistor's ohm load doesn't fluctuate like a speaker does for example by: coil temperature, box rise, humidity, vehicle dimensions, etc.. But if the speaker is going to fluctuate why would you use that number? lol..the whole situation is useless.
Amazon.com: Hand Held Digital Oscilloscope-40MHz: Electronics