I'm trying to do some calculations for possible energy yield of a fission reaction, and I need a general formula for the energy requirements of fissioning an atom
An approximation is the Bethe-Weizsäcker formula. There you can get the approximate binding energy for the input and output nuclei and then compute the mass/energy difference.
A more accurate result would be gained from measured data of the weight of the nuclei. Then take the difference in mass (be sure to include all free neutrons produced) and convert that into energy with $E = mc^2$.
All this will give you the net energy produced by the reaction. It does not tell you how much energy is needed to initiate that process. For that one should perhaps look into the energy levels of the input nucleus and see what exited states it has. Computing this will be hard to impossible, I think.