That's a good question and I'm sure the Department of Finance is busily trying to figure out what assumption they should make. It's hard to find what assumption they were making in the last budget, but when you track the price of oil since 2010 it's fluctuated above $100 a barrel. Now we're at roughly just over $80 on Brent and $70 on West Texas, so that's a really tough call. I would probably err on the prudent side, to be frank.
On October 28th, 2014. See this statement in context.