Scanning quickly, it seems that the "75 years" is based on calculating "this is how many pages of documents that have been requested" and "this is the minimum p

Scanning quickly, it seems that the "75 years" is based on calculating "this is how many pages of documents that have been requested" and "this is the minimum production rate we can sustain [for this individual FOIA request, noting that there are ~400 other FOIA requests also pending]". The FOIA request apparently consists of 329k pages, and the production rate is 500 pages/month, and dividing the two numbers indicates the "55 year" rate that was originally suggested. I didn't continue to see where the numbers for the 75 year revision comes from, but that gives you the general picture.