You make one item,its a one off,and so on and so forth,thats how i was taught,many years ago.Why? It doesn't make sense to me at all. I'm thoroughly befuddled.

1/3 of 1 = 1/3
1/3 off 1 = 2/3
off = subtraction from the sum no?
1/3 of 1 = 1/3
1/3 off 1 = 2/3
off = subtraction from the sum no?
It is off, don't ask me why, but off is what is used in our office as well.
Ditto, seems to be an engineering thing as when I asked my dad he said the same.....
@Voltar and others
Not sure why people are acting like this.
It's a reasonable question to ask and I've never heard the word used in this context. My first thought was that it should be "of", but I suppose it makes sense.
Always took it as meaning "can you run one copy of this off". Such as 4 off this, 3 off that. I'm talking purely in a CAD environment and it's certainly how we used it, in fact if anyone wrote "2 of" they'd be pulled up for it.

Wow.
Let me get this straight, you make copies of something and you say [numerical] copies off [item]?