I suspect (but have not attempted to prove by calculating) that fuel usage in nebulas is unaffected by your actual forced speed. In other words, I think that fuel consumption acts as though you're moving at (for example) 2000 kms instead of the imposed nebula speed 800 kms.
The reason I think so is that I have a system with a level 26 nebula. I sent my FAC survey ship off at a slow rate, but figured I'd get there eventually. In the month or so it took to move the ship the ship was running out of fuel. The time frame was normal for my survey ships in clear space, but I figured I would be able to survey for the year or so it would take without too much problem.