Originally Posted by Mr Bill
You might be onto something. This is where having a wealth of different answers can be confusing, I saw Hoya's and B+H contradiction and didn't know what to make of it.
Film type, light source, meter type, how you meter, what you want. All play a part in black and white filter factors.
Film type: Depends on the film you are playing with. Some differences are significant and might be several stops (Ortho/IR). Other films (Technical/Tabular/Traditional) differences are subtle but might lead factors to vary about a stop .
Lighting: (Tungsten/Daylight/Open Shade). Could be a stop just as you pointed out Mr Bill.
Meter type: Have you seen how super-responsive a selenium meter (Master II) is to Tungsten. This is a documented reason why manufacturers used to recommend lower EI in Tungsten... With tungsten less blue, less actinic... And with meter jumping wildly to the light bulb... Old-timers had two problems going opposite directions - so something had to be done. They recommended lower EI in Tungsten. Now we have better meters.
How you meter: Simply does your setup or habit make you meter through the filter or do you meter without filter?
What you want: If you want detail in shadows then "Hutchings" factors help you obtain that.