I would say each level of brightness you have listed would be 1WPG
Very low <1wpg
1 < low < 2
2 < medium < 3
3 < high < 4
4 < very high < 5
Generally you want to be within the 3-4 WPG range I believe, as this will grow most plants but won't be so much as to stress the plants or be overkill for nutrient uptake.