The high rolling software contractors I know use "billable hours" as a random meter stick to charge whatever they feel like. I know a guy who has more billable hours than waking hours across the week across three contracts. The customers know and don't care, because he gets things done. I've even heard people complain about clients over beers and say "because they were being so stupid I added 8 billable hours"

charge what you want. Billable hours are to make the departments approving the spend feel comfortable. What are they going to do? Find someone else who can build a complex OLTP system in 2 months?