Likely it's made up and no one can find the answer to it. Who even knows the calender used to calculate such timelines. I don't think Russell or Rutherford et al sat down with a couple rabbis to get a clearer view of Jewish historical events, calendars in use, and dating.
I'll have to check out Russells writings to see if he may specifically allude to the calculation. Ugh! It's like reading the ramblings of a 6 personality schizophrenic.