Consider my lovely Nizo 561 Macro.
At 24fps the shutter speed will be 1/60s
And at 24fps with 200ASA Tri-X in the camera and on a sunny day the f-stop is about f/16 (I just checked this).
So the Sunny 16 rule says to correctly expose a sunny scene with 200ASA film, we need f/16 @ shutter speed 1/250s and this is 2 stops under what my Nizo is actually doing, f/16 @ shutter speed 1/60s.
On page 1255, The Case Against Professional Super-8, it says,
A Super-8 camera/lens using the beam-splitter system will lose better than one stop and sometimes as much as two stops in the lens/finder optics.
and thats it!
The two stops are lost in the viewfinder which splits off some light so you can see whats going on, and also in the optics and the light-meter.
So if your manually setting the f-stop on your Super 8 camera, you may well have to add between 1 and 2 stops to your f-stop to get an adequate exposure.