As I have indicated many times before, assuming a set number of successful hits is a stronger form of assuming a set number of attacks. It is essentially assuming (in this case with 50% hit probability) a set number of attacks, exactly half of which will be hits and half of which will be misses.
Not in the least. It assumes nothing of the sort.
It matters not if there if 5 attacks or 10 attacks. If 4 of them hit, how good is Shield? If 2 of them hit, how good is Shield?
Conversely, if 2 of them hit, how good is Second Chance? How much damage does it stop?
You are really getting hung up on standard probability and forcing the question to relate to number of attacks.
That is one approach, but not necessarily the best one when considering game mechanics which only occur on a to hit.
If we were talking a +1 to AC game mechanic, then it affects every single attack against AC. There, it is important to consider attacks because the mechanic affects attacks.
Second Chance and Shield do not affect the initial attack roll. They come into play AFTER an attack is determine. It's a different type of problem.
Shoe horning it into standard attack equations is not quite valid.
Using your method, 4 hits= 100% chance you get to use Second Chance. Using my method, 8 attacks at a 50% chance to hit= 1- (1-0.5)^8= 99.6% chance that Second Chance can be used. Done.
And this illustrates two answers to two different questions.
Just like the other two equations do.
I'll get back to what is important for these two powers (and you stated that it was the important metric yourself). What happens when one gets hit and one of these powers can occur. How does it change the damage?
It matters not what happens if the attack misses. That's outside the domain of the scope of these two powers. Instead of using the Set, you are using a Superset.
If you notice, this was never the question I was answering. As my very first post on the subject indicated, I'm working with average damage prevented in an encounter.
And, that's fine. You answered a different question.
But at the same time, I did not claim your math was incorrect for what you wanted to solve, you claimed that my math was incorrect for what I wanted to solve.
Going back to this last example:
Miss and Miss = 0%
Miss and Hit = 30%
Hit and Miss = 30%
Hit and Hit = 51%
For me, what is important is that Shield is effective in that encounter 30% of the time on one hit and 51% of the time on two hits. That's what is really important to a player.
Not that out of two attacks, Shield is effective 27.72% of the time. Although related to the important question, it misses the mark slightly.