If you're altogether certain, you know it for a fact. Since you're altogether sure that your calculations are correct, go ahead with the experiment. Please wear your goggles just in case.

Never use “all together” (meaning “all in one place”) when you mean altogether (which can also mean "all things considered"). It was risky to bring us all together again after so many years, but you have to admit that altogether we had a good time. Altogether, it could have been worse. It wasn't altogether bad. I suppose we won't do it again next year.

Definitions of altogether
  1. adverb
    to a complete degree or to the full or entire extent (`whole' is often used informally for `wholly')
    “it was not altogether her fault”
    “an altogether new approach”
    synonyms: all, completely, entirely, totally, whole, wholly
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    part, partially, partly
    in part; in some degree; not wholly
  2. adverb
    with everything included or counted
    altogether he earns close to a million dollars”
    synonyms: all told, in all
  3. adverb
    with everything considered (and neglecting details)
    altogether, I'm sorry it happened”
    synonyms: all in all, on the whole, tout ensemble
  4. noun
    informal terms for nakedness
    “in the altogether
    synonyms: birthday suit, raw
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    type of:
    nakedness, nudeness, nudity
    the state of being without clothing or covering of any kind
Word Family

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