I have a Yashica Electro 35 film rangefinder camera. I usually load some higher sensitivity film so I can easily take pictures indoors, but now in summer I ran into trouble shooting outside on several occasions. There was simply too much light. I figured that I could use an ND filter to deal with this; the problem is, Electro 35 has an exposimeter on the body rather than through the lens (unlike, e.g. Minoltas). This means that even with an ND filter, my exposimeter will still measure the scene without it.
So my idea is that I could just lower the ISO setting on my camera to compensate for the drop in light caused by the ND filter. The problem is I have no idea how to calculate proper values and I didn't seem to find anything online.
Is this idea total nonsense for some reason, or is it there a way to get to the numbers? How strong of an ND filter would be needed to successfully work with ISO 800 film, but have the camera think it was ISO 100?