The number of jobs to run for an optimal compile time can be quite confusing. If none of the files you are going to compile are cached it is alright to run a lot more jobs then usual as a lot of them are going to wait for the disk I/O. After that twice the jobs then you have cores is mostly appropriate.
According to Con Kolivas's benchmarks, with the BFS scheduler you just do make -j [numprocs] for best results. I can't recall if he was accounting for disk cache, though.
