I bought 400 ISO (color) for my Leica IIIf, but found out the highest setting is 100 ASA/80 Weston. What should I keep in mind when taking photos regarding other settings? It’s going to be rather cloudy where I’m shooting this week.
I don't think this model has a built-in light meter. The dial you are referring to is just a reminder indicator. You set it when you load film and then weeks later when you pick up the camera, you consult the dial setting to remind you what film you have loaded. Use a hand-held light meter or the "Sunny f/16 Rule" to set your camera.