I'll try. These are two different types of words.
To be wide of the mark - of is a preposition here, similarly to "to be close to the mark" (which is the opposite).
To be off the mark means you're nowhere near it.
off + noun - is often used in set phrases.
off (or wide of) the mark
1A long way from an intended target:
‘most of his shots went wide of the mark’
2Incorrect or inaccurate:
‘past demographic projections have been way off the mark’
1British Slightly unwell:
‘I'm feeling a bit off colour’
2Slightly indecent or obscene:
NB verb + off is usually a phrasal verb.