I am no expert, and I think that statement is very subjective, depending on your eyesight. If, for example you pointed a high output halogen light at the work before welding, I could weld okay with a 10 shade. If its just everyday light, I need an 8 and even then I need to focus hard. I try to get some light on the work if I can, but I am most comfortable with 8.
Also, 'light welding' is subjective. I use an 8 if I am welding trailer hitches together on deep penetration, or if I am welding light sheet metal on very light penetration.
I guess if I had anything useful to say, it would be.. Be able to see the joint you are welding, but just barely. Don't guess where you are going to stitch next, but don't make it so you can see it too easily. This is why I am so happy about cheater lenses. Magnification lenses allow you to get right down to the closest you can be so light is less necessary. Since I got my Viking 3 helmet, welding has been a whole lot more fun. Now it isn't about grinding off all the welding that was not even necessary, but rather getting good penetration and higher quality. Night and day! Hope this helps!
