The basic rule has always been to use the darkest shade that will still let you see what you are doing. Virtually any good filter will block all the UV and IR that does the most damage. Everyone is a little different as to the best filter level for each job. Also this does change with age somewhat. The Navy did some tests many years ago that showed only 18 year old kids have the best night vision because their pupils can dilate the most. They used this research to design binoculars for night watch on ships that would gather the most light for that size pupil.
Long arc, short arc, heliarc and in-the-dark!