Perhaps the quality, or rather type of lighting plays a part in this. For me it was all about a zone III placement in a scene recording as zone III regardless of filter being used. Shadows predominately have more blue light so you would thing that the use of a red filter such as the #25 would block lots of this blue light requiring more exposure. Hence the filter factor of 8x....but my testing indicated that that was too much compensation. 4x was adequate. My approach was first sensitometric, making in camera exposures of a sheet of film in my darkroom using a light balanced for day light. I used Gordon Hutchings' Zone Board idea as the target. After that I try it out behind my house. I shot the same scene under overcast and clear blue sky using my data and the negatives look and print very nicely. Seems like a lot of work but I've learnt a lot in a very very short time.