Well, the first part of it (engine reserve) is fairly simple. Determine how much a new engine, factory reman, or overhaul costs. Multiply that times the number of engines on your airplane (two in all your examples), then divide by the number of hours to TBO. Save that much per hour to pay for the overhaul.
Next, determine the cost of any upgrades you may need/want ... avionics, TCAS, RVSM (the latter two more oriented to the King Air), or whatever else. Factor in the cost of annual/100 hour or phase inspections (shops should quote you a "no squawks" price). Then, you'd need to make some allowance for unscheduled maintenance. I don't know of any reliable formula to account for that one.