A way to calculate orbital energy of an electron
You are correct that the ionization energy of an electron depends on the orbital. There is no easy formula, because the electrons in atoms are governed by quantum mechanics and for multi-electron atoms, the equations are un-solvable algebraically.

That said, there are some reasonable tabulations, which have been verified by calculations and by spectroscopy, e.g., http://www.colby.edu/chemistry/PChem/notes/AOIE.pdf (from a quick Google search).

Both ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy can be used to eject electrons from various materials, and you can thus identify elemental composition (and other properties) from the known orbital energy levels