Much simpler than I expected
Leave the layer modes as "Normal" (not multiply, or anything else), and just set the weights as you prefer. A true average of the frames would be as you said:
Layer 3: 25%
Layer 2: 25%
Layer 1: 25%
Background: 100%
But it's quite nice being able to customise this I have found. Being able to give a little more or less weight to different exposures which are potentially of better or worse quality, longer or shorter exposure, etc.
I've been working most with 3 frames:
Layer 2: 33%
Layer 1: 33%
Background: 100%
It's amazing to see with only 3 just how huge the noise reduction is and increase in clarity when hiding/showing each of the layers, it's quite fun
Quite powerful to also be able to clone between the layers before flattening them (to remove dust/aeroplanes/etc).