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Just a quick answer, but I am not sure to understand your problem.
The filter correction factor has no relation with exposure but with film. In older datasheet provided by Kodak our Ilford it gives you the correction factor to apply to the film. For instance, if you use a wratten 15 filter, the correction factor is not the same for the TriX, the Hp5 or the TMax films. You cannot appreciate the correction factor with a meter. Remember what they teach you. First compose and meter without the filter. Then screw the filter on and apply the correction factor to your exposure. Even with a spotmeter, you cannot meter the correction factor because the cell do not react in a same way that the panchromatic or orthopanchromatic films.
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