I train welders days and nights. Refer to the following pub for info. For arc welding most helmets come with a #10 lens. Adjustables 9-13. The lens darkness is a function of light intensity(amps) and duration. As we age, it can become harder to see the puddle. Eliminate light reflecting into the back of the helmet. I also have some of my older students (me too) use halogen shop lights to illuminate the area. It also takes a lot of practice to read and see the puddle. A 3 or 5 shade is required for oxyfuel work. A 9 or higher for plasma torch work.

http://www.millerwelds.com/pdf/safety/m199776c.pdf

Jim

Good luck with your project!