Thx Rossco. That makes sense. I will put a couple in. Matches in the rough range I was thinking. That's the down side of running petrol. You need more and it's more flammable to carry. I got used to it in Saudi, took out the back seat and had 16 in there on my last big trip.
@Winnie, the Jack only takes 85 litres. So doesn't give too much to play with. Putting one or two in will make me comfortable not to stress about running out.