two other things that effect filtration are:
1. the density of the negative
2. the color casts caused by the things that light bounces off.
1. keeping your exposure constant under the same light (read that, don't use autoexposure) will keep your printing filtration constant.
2. take a portrait with much of your light bouncing off grass and you'll have to correct to get good skin tones, move ten feet onto a sidewalk and you'll have a significant filtration change, even though the sunlight has remained constant.