My experience, in a quite different field of endeavour, is that one must be careful to set the rate high enough at the outset. Long term customers don't like rate increases, but oddly seem fine with what I think of as "higher" rates from the outset.

As a specialized expert on the Linux kernel, I generally charge my best guess at 2X-3X the hourly rate of a customer's best full-time employees.

Cheers.