It reaaly depends on what metal you are welding, and the proccess. For safety sake use it as dark as possible, but we all tend to cheat in this area! when doing steel you can usually get away with 9 or 10, definatly when using tig. when doing aluminum, you MUST set it higher! it reflects way more than steel. You might not notice anything at first , but your eyes will hurt at the end of the night if its too low of a setting. Also watch for tack welding, some lenses are not able to adjust to the quick bursts of light when you are doing repeated tacks. The high dollar hoods have settings for this.

The best thing i have found , is like denny said, put a spot light on the area to brighten up the place, then set your lense on at least 10