Having worked in a manufacturing company that always used '100 off please mate', I can say that in industry off is accepted.
Obviously gramatically it's 'of', but not everything is always that simple. There are probably loads of similar examples (none spring to mind!).
I'd use "of", but my dad will write "off" (pronounced of though, I believe) and he works in engineering.
Hmm, interesting. D'you think that's just because someone started using 'off' incorrectly and it was 'passed down'?

