Ok here is what is required for the 100% achievement:
Spaceship Parts: 50/50
Letter Scraps: 50/50
Knife Flights: 8/15
Under the Bridges: 25/50
Stunt Jumps: 25/50
Nuclear waste (30), Submarine Pieces (30), Epsilon Tracts (10) & Hidden Packages (12) are NOT needed for 100% completion but may be required for other achievements!
I will have a thread with most of the collectibles hopefully by tomorrow night.