Tuning amp gains with DMM?

So after trying to tune in my Rev410's this weekend I figured out really quick I need another way. hey are so loud I don't feel as if I can accurately tell when they are distorting. I've been doing some research on tuning using different frequency sine waves and reading the voltage of the amp output with a digital multi-meter. I think I understand everything, but have a few questions...

For reference the equation is Voltage = square root of watts * ohms.

1) All the reading and videos I've watched talk about doing this method on 2 ohm loads. The Rev410 runs on a 4 ohm load so I'm assuming I need to multiply the amp RMS by 4 instead of 2.

2) I also read that if the amp has 2 gain controls to treat it as 2 separate amps. EX. If the amp is 100W RMS by 4-channels for a total of 400 watts, divide the total RMS by 2 to get your voltage calculations. 400RMS / 2 = 200RMS * 2ohms = gain voltage.

3) So given these scenarios, the Syn4 running a 4 ohm load should calculate as:
400W RMS * 4 = 1600/2 = 800
Square root of 800 = 28.284 VAC at the amp outputs.

4) Another way to think about it is the Syn4 when bridged only uses one gain knob. So do I calculate it like a 2 channel amp then?
400W RMS * 4 = 1600
Square root of 1600 = 40 VAC at the amp outputs.

Is this correct?