don't think I could bothered with working out a price per litre on top of an hourly rate, either do it at the hourly rate or quote your preferable hourly rate, or look at the job and give him a price, I have had these sort of dealings in the past and it soon becomes apparent why they are well off and well dressed, because they don't like paying for things and always look for a bargain, then they start talking to you like they employ you sod that, just give him a price