Averaging images using layers

[Bob]

I am working on a project to measure the various components of light in a night scene and need to stack and average multiple images.  I am doing this using layers in GIMP 2.10.8 and hope to reduce the opacity of each stacked layer; eg 100%, 50%, 33%, etc.  However in a simple test using just two layers (Black, 0.000, at 100% opacity and above that White, 1.000 at 50% opacity) the resulting "visible layer" pixels had a value of 0.737 not the 0.500 I had expected.  I am not sure what I am doing wrong but would appreciate any advice on how to stack say 10 photographic images which gives equal weight to the pixel values in each image?