No. It "just so happens" to work out (theoretically) when the power you are dividing by is a 1ohm power figure. Just plugging the desired impedance into the denominator will be inaccurate for most any other impedance.
You could apply basic ohms law. It won't be perfect (well, it would be in a perfect world //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif ), but it'll give you decent approximation of what to expect.
To show how the "formula" you quoted would be wrong if it were at any other impedance, let's say the amplifier did 1332w @ .5ohm and we were wondering what it would do @ 1.34ohm.
If we were just to divide the given power by the impedance we were wanting the predicted power figure for, it would give us;
1332/1.34 = 994w
Using ohms law;
sqrt(1332*.5) = 25.81V
25.81^2/1.34 = 497w
If you wanted to use the actual formula Rashaddd posted, it would be;
Given power/(desired impedance/impedance of given power)
For example;
1332/(1.34/.5) = 497w
You can see why this works eloquently when we are given power @ 1ohm, because any number divided by 1 is itself....so it cuts out the actual math of the denominator. But I don't like that formula because it doesn't show you the mechanics of ohms law that are at work.....
This later formula is probably the one Rashaddd used, but without showing you how the denominator was actually arrived at.