He assumes that an active person would need 3,300 calaries per day (note: you can almost halve this if you are sedentary). Because few plants can provide enough calaries without making a person sick, most of these calaries would have to come from meat. He also produced a neat table to summarize how many animals you would have to kill and eat to meet your requirements:
|Type of Animal||oz of Meat/Animal||cal/oz of Meat||Total cal/Animal||Animals/Day|
And for plants:
|Type of Plant||cal/oz of Plant Material||Pounds Per Day Needed|
|Cattail Root (Rhizome)||8||26.5|
He goes on to discuss other considerations, including the "opportunity cost" of hunting/gathering, legal considerations, etc. Read the whole thing.
(H/t The Woodpile Report).