Gains

hi, if i where to buy a bigger amp then my speakers can handle i would use the speakers rms in the formula to find the proper volts that i need right.

Depends on what your goal is

If your goal is to send the speakers their rated power when you hit the headunit's clipping point, then yes. If your goal is to set up the system so that the amp clips at the same point as the headunit, and you just use common sense to keep from blowing the speakers, then no.

I always buy amps rated higher than my speakers RMS

ok so with this formula which should i use if i don't want to blow my speakers.
( don't mind the numbers) ( the speakers rms or the amp's)
The formula is sq root of Watts x resistance(ohm load)

400 watts by 4 ohm is 1600 then we find the sq root of 1600 which is equal to 40

We now know that 40 volts is equal to 400 watts.

This 40 volts will be found with the DMM

yes, theoretically

that's the gist of it ....... certainly a good place to start, just remember that you're output will be pretty low doing it like that.

will the speakers i want to use are 110 rms and the amp is 150 that i want to use.

that shouldn't be a problem if you watch the volume