The thing is you keep talking about value, as if the fight was a flawless execution of a series of casts until the boss die. As if the boss was a simple mathematical equation. And you may be right to some degree, but as you mentionned, RNG exists, and each try is different.

I will even add that each player is different, in terms of gear, in term of teammates etc

You try to rationalize it by putting away any kind of randomness and crunching down the numbers. Your method has its merits, your guide is a proof of that.

But I prefer the "realistic" approach, one where your raidmates don't always play to their full potential, where the RNG can sometimes be cruel, and where each try is a new series of decision.

In that approach, I like to imagine what happens when things go south..