hahah
really there is no clasical formula except the one I gave you at the begining.
All other apporaches are restatement of I said.
In Qunatum World things get solved non-sequentially. Multiple Dimesions get involved in its calculation. Making the solution fast to very fast depending upon the ability to make the ideal mesurement after time T out of ideal experiment P.The Time T is very small as compared classical calculation time t.