Each full grown hen eats from 1.8 to 2.2 lbs per week
( mine are medium sized, use 2.0)
Total food for 20 hens x 2 lbs = 40lbs per week
Pasture grazing of bugs and greens is estimated to use 10% to 25% of total
This estimate is low because factory chickens do not move, but pastured chickens do. They therefore use more energy which requires more feed than the estimate for factory chicken.
.25 x 40 = 10 lbs. 40 - 10 = 30 lbs of feed still required
This is all based on what I have found after many hours of trying to find research about free ranging. My own experience with three pullets is that they still want their feed even though they like (demand!) going out for free range time. I have a lot of bugs in florida, enough greens that they eat what they want, but they choose to go back in the run for store food. I give all three (11 weeks old) a total of one 8 oz cup of food, plus 1 to 2 hours of monitored free range and they are all growing well without looking fat, are healthy, and have good feathering.
A good idea is to create piles or rows of plant litter to attract bugs for them. Once they found the first pile they were in heaven!
Mine also went mad over grass seed in my yard area after it went to seed and the seeds started to get ready to fall, so letting pasture go to seed is also beneficial.