I read through the conversation on this question: Is there a way to render a single frame using network rendering in cycles. I tried the image stacking method that was suggested and got pretty good results. I rendered my image five times, each with 150 samples and a different sampling seed. I then rendered the same image with 750 samples to compare. (In case you're curious, the 150*5 stack had slightly more noise than the straight 750 image, so you are sacrificing some quality by splitting up the work in this way)
Then I wondered how much of a quality increase I could get if I added the 750 image to the stack, which brings me to my question. How can I reduce noise by image stacking if the images don't have the same number of samples?
So my first five layers have opacity 100%, 50%, 33%, 25%, 20%, respectively. These are the layers with 150 samples. If I wanted to add my 750 sample image to the top of the stack, what would be its opacity?