I rigged a trigger pull gauge which says 300 grams force at the spring to move arm fully to the rear. Just in case you want to apply Hooke's law and get it just right. [8D]

edited addition: You can try to test the Zip drive just to see if it otherwise works by using some transparent tape on the topside to manually secure the zip drive media shutter in the open position before you insert it. I have not tried this in a springless drive, but unless they have some sensor interlocks verifying the arm goes fully forwards it should work.