Advice is given in the standards.
In Europe, refer to EN 169 2002 'Filters for personal eye-protection equipment used in welding and related techniques'
In this document, Table 4 relates process and current levels with grades of filter required to give protection. This standard also contains comments on the "Scale number of filters to be used by welders' assistants."
In addition, EN 175: 1997: 'Personal protection - Equipment for eye and face protection during welding and allied processes,' gives guidance on the system supporting the filters.
EN 379:2003 Personal eye protection, Automatic welding filters.