Sunny 16 rule

The Sunny 16 rule states that on a nice sunny day, you can set your lens to f16 and your shutter speed to 1/ISO and get a very accurate exposure. It totally works.

If it is a little overcast but still bright, then you can use f11 instead.

The Holga has a fixed aperture of f11 and a fixed shutter speed of roughly 1/100th of a second. So how does that apply? Well, if you are going to go out and shoot on an overcast day, then using an ISO 100 film would be perfect. Of course, you can't change film from shot to shot. Let's say that you use an ISO 200 film instead. On that same overcast day, the film may be overexposed one stop. That is okay as print film is very tolerant of overexposure. Slide/transparency film doesn't tolerate overexposure. What if all you have is ISO 400 film on that same day? Go for it. That is two stops overexposure (assuming a perfect world). It will still be fine. Worst case scenario is that some of your highlights will be blown out.

If you plan on doing double exposures or more, then you should use ISO 100 in this case. Otherwise you risk grossly overexposing your film with cumulative exposures.