Personally, I have only ever heard 'wide of the mark'.
However, as 'wide off the mark' also makes sense, I wouldn't be surprised to learn that some people say this, too
٢٥ يناير ٢٠١٥
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The correct answer would be wide of the mark.
Off would be used in sentences such as the following
I switched the TV off. The child jumped off the swing
Hope this helps