portredleftonshelf,
Since no one comment on this subject, let me try.
Yes, there are some program around to assist in calculating of the hourly rates (not sure if it is in an excel format or not), which usually design for the GA and the Corp Market. The one for commercial is difficult since more data required to form the calculation.
But the main factor is work out your estimated hours to be flown annually, then the rest are Pilot Salary, Insurance, Hangar, Annual Inspection, Fuel, Oil, Routine Maintenance, Engine Overhaul etc.. But you may found it earlier to work out the figure yourself.
Anyway, hope the above would be helpful to you.