And I still don’t understand the probem with the UnsignedByteType Images and the Image Calculator. The results are fine, where are the numerical errors?
See my answer here where I added an example workflow: How to merge images in different rows? - #3 by gab1one
Regarding the thresholding, you can experiment with all the different methods in the Global Thresholder and the Local Thresholder nodes. Some of these methods are much better at handling complicated images, than a simple mean threshold.