Do not think in hours. You are not a worker in a factory. Consider the value you deliver. When you deliver twice the value in the same time you did a year ago charge twice as much.
And some areas like law are very hourly-based. The one time I did some legal work it was strictly based on time. (At a very good hourly rate including travel time.)
You mitigate the risk of getting paid not enough in case the hours spiral out of control while still getting paid a lot when you outperform expectations.