Yes, on second thought, I think you’re better off calculating conversion rate using a a ratio of reaction rates as suggested by @gridley. Namely, the numerator (fissile material produced) can be calculated by summing the tallied (n,gamma) reaction rates for each fissionable nuclide, and the denominator (fissile material destroyed) can be calculated by summing the tallied absorption reaction rates for each fissionable nuclide.