Setting gain with a DMM question

[FamousMassacre 10+ year member]

If I'm using a dmm and have to do the square root of watt times ohms, do I use the "official" rms or the actual rms if it is under rated?

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[maylar 10+ year member]

I prefer to use "rated" or even a bit less. If you have an o'scope you can push it up to the limit but with a DMM I'd keep to rated.