I couldnt load the link from my phone but basically when using filtered light there will be an increase of exposure time that will be needed to get more density. This varies from paper to paper and the correct filtration chart for your variable contrast paper should be included in your box of papers.
It is best to make test strips until you get familiar with your head and paper brand. Grade two filter is usually equivalent to standard white light in contrast, going up increases contrast, going down decreases. You can go both ways for split printing.
Extension factor has to do with the relationship between the distance of a light source and intensity of it. The further the light, the less intense, such as when raising the head up, you will need longer exposure.