Im not in the jet washing business, but I am also not a fan of day rates. I think that you can easily sell yourself short.
For example, what you might think has been a good days work, may not be to your customer. Inspect the job (when you know what it is) work out your costs, antisipated time that it will take and submit your price. Simple.
Your pricing must reflect others in your area, especially if you are new to the job. You cannot go in top dollar at the beginning, as your pricing should reflect you experience and knowledge.
Best regards
Paul