By "other tools" I mean those tricks one can use with either a ruler or other standard item that you include in the scene, and then read the exposure adjustment off of the image on the ground glass.
Or you could use that fancy math you are trying to avoid
Believe it or not I'm actually still confused, I don't understand what helps me about putting the ruler into the scene, I'm just not following, also I wouldn't think that reading a spot meter on the back of the groundglass would give me an accurate reading because the light is already diminished?
Just really not following you there
~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller