the zone system and the use of filters

i,ve been using the zone system for over a year now and I´m happy with the results so far. Lately, i´ve been trying to use filters (red-cokin 003) basicaly and it gives me trouble calculating the less amount of light reflected by some objects and the extra amount of light reflected by others. There seems to be no way to exactly calcutalte the diference of what I see with and without the filter and therefore placing them on the zones. Marçal from barcelona, spain. thanks

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