There is, to my knowledge, no practical means of measuring "dynamic visibility" along the glide path. Therefore we are stuck with the 3 measures in place: prevailing visibility, RVR, and Vertical Visibility.
In low-visibility situations, RVR is the most important, since it is on the runway that your visibility is most critical. That is recognized already in the rules for Cat III approaches -- you need not see the runway before landing.
In ALL cases for Cat I and II ILS approaches, the Decision Height is, in reality, a single point in the sky where the glide path intersects the Decision Height altitude. Either you see the runway at or before that point, or you do not. Respectively, you either continue to a landing from that point, or you initiate a go-around.
Visibility depends on more than precipitation. Lighting conditions (e.g., contrast) are a significant factor. For example, it is MUCH easier to pick out approach lights at night because they are so much brighter than the background.
Bottom line is, there is no need for any "reduction factor". Either you see the runway at the DH or you do not. If you want lower minimums, you get certified for a Cat II or Cat III approach. If there is no Cat II or Cat III approach for that runway, it is because approaches in lower visibility have not been deemed safe.