It's more of a physics problem I think. Work with power*time, divide by the efficiency to get thermal energy, divide by the energy released per fusion reaction, multiply with the mass of tritium, get the lithium to tritium mass ratio from the breeding reaction. You also need some approximations on the way - the energy per reaction will depend on the reaction rates of tritium breeding, and lithium-6 and lithium-7 have different masses so we need that ratio there, too.