Your mixing is not matematically accurate
The mix factor of the last node should indeed be set at 0.2 to get 20% of the last image.
However, if we look at the second to last node, we need to set the factor in such a way that x*0.8 = 0.2 or x=0.25. Another to see this is to think that the factor of 0.8 needs to be divided equally between the 4 nodes remaining, hence 25%.
It's the same thing for the third to last node, we need to divide it equally between the three remaining nodes hence 33.33333%.
Finally, the last mixer needs to have it's factor set to 50%.
Hope this helps!