To calculate something you must start somewhere, ie from known information
Do you have an example ?
Is this a book exercise involving ionisation in solution where the heats of solution etc are given and the missing reaction heat to be calculated is th ionisation energy?
Ionisation levels for atoms are normally measured, not calculated, but to calculate from first principles is known as Hartree theory where the difficult equations for multi electron atoms are solved by numerical methods.
Did you want this ~ it is well beyond high school level. ?