What is the formula that geant4 uses to calculate the radioisotopes produced in the second column? Can this calculation be done manually?
There is no “formula”. That is a count of the number produced in your run. Ifg you change the /run/beamOn argument, then you’ll get different numbers in column 2. However, the ratio should be statistically the same across runs.
I don’t understand your question. The Monte Carlo simulation you’re running “is” the calculation. The radioisotopes are produced randomly, depending on the particular interactions in each event. You’re welcome to use a pen, paper and some 10-sided dice to reproduce the simulation manually, but that seems like a lot of effort when the computer is already doing it.