The way that exposure ring on all or at least most of the 200+ series pack film cameras works is that it just slides what is essentially a graduated neutral density filter in front of the CdS cell (the "electric eye" which I'm pretty sure is CdS based on this manual I found) to control the amount of light hitting the cell. So set it to "overexpose" by +2EV and internally you're literally just sliding a -3EV ND filter in place. Set it to -1EV and you get 0 ND filter.
The other thing is that the little PC sync plug for the flash has this other funky slit next to the PC connector for the old style flashbulb flash. When the flash plug gets inserted into this, a little scrim (opaque screen that blocks some percentage of light, similar to an ND filter) gets placed over the "eye" in addition to the ND filter strip. I'm not sure what this is about though, perhaps something to do with the slow-burning disposable flashbulbs. When I hook modern strobes up to it it works fine, but the photo gets screwed up if I stick something in there to put the scrim in place. (Or maybe inserting something moved it out of the way... don't remember.)
I think it's possible that low battery voltage could have some impact on the exposure time because that CdS cell is basically a variable resistor that changes depending on how much light hits it, and it appears that the shutter closes after it lets a certain amount of charge (mA/sec) through. If you have less voltage though then it should let current through more slowly for a given amount of light and cause a longer exposure, but the thing might have a voltage regulator to prevent that. (I'd hope so.)
Anyway, I also have to set the thing to +2 stops brighter to get a proper exposure. My theory as to why is that plastic gel-type ND filters do fade over time (which is why lighting gels have to be replaced every so often). So the gel fades out and the -3EV ND filter becomes like -1EV. If 0EV (no filter) is the "1 darker" setting then -1EV ND blockage would make the -3EV ND "two ticks lighter" filter turn into a -1EV ND filter, making "two ticks lighter" become what "no ticks lighter/darker" should be.
I usually use ISO 100 FP-100C film and set the film sensitivity knob to 75 on my 250, so I'd need to lower the exposure a bit anyway. With 3000 film you could set the wheel closer to 1500 (if you have that setting) to give it another stop of light which should allow you to turn the exposure knob down one tick.
I also thought about just cutting up little bits of this -1/2EV neutral density gel and taping a few of them to the camera. If I need to raise the exposure more then I just tape however many of them I need over the eye. That effectively gives you manual shutter speed control, limited only by the number of ND gel strips you have and what density they are.
A red marker may work, but since it's basically blocking cyan to reduce the amount of light, the amount of light it absorbs is going to change depending on the color temperature of the light. At 6500K sunlight outside it will block more light than at 2800K with incandescent lights inside because there's more cyan-range light in 6500K daylight for example. A lot more toward dusk when the color temp is like 9000K.
(Id post a photo showing the little ND filter slider but it looks like I forgot to take photos of it when I took the thing apart.)
Other than actually getting an ND filter gel (you might be able to buy a small one somewhere made for use on speedlights for like $2) I'm not sure what the best hack would be for making your own.
