I don't believe my post was insulting, but feel free to think otherwise.
You're combining percentages wrongly - you're using addition/subtraction where you should be using multiplication.
As a simple example, if you take an initial output, through one stage of 50% efficiency, then another stage of 50% efficiency, you don't end up with 0% efficiency (1 - 0.5 - 0.5 = 0) - you end up with 25%. ( 1 * 0.5 * 0.5 = 0.25 )
I have no idea if your actual figures were correct - I am not a thermodynamics expert - but using the maths correctly, you end up with more like :
0.33 - 0.5 = 0.28
0.28 * 0.9 = 0.252
0.252 * 0.9 = 0.2268
So about 23% - not 8%
Note: I have no idea if the 23% is actually accurate in terms of the science, it's merely the result of applying the correct mathematical process to your figures.