A Fell Cleave is not actually a 520 potency gain. Because it happens on the GCD, you gain 520 potency but also lose the potency value of the GCD of the GCD it replaces, as well as any resources that you would have gained from that GCD. As an example, if you replace Heavy Swing, the potency gain is

50 gauge = 520 - 160
50 gauge = 360
10 gauge = 72 potency

Butcher's block is unique in that it actually has more raw potency than Storm's Path. So you actually gain 30 potency for using it, but lose 10 gauge. That's where the 42 potency comes from. And that's the very worst case scenario. If you replace any other combo action with Fell Cleave, the value of Fell Cleave drops off because you're losing gauge and potency for it. If you truncate Storm's Path, for example, you have

50 gauge = 520 - 280 - 20 gauge
70 gauge = 240
10 gauge = 34 potency

So if that's the Fell Cleave we're talking about, then Butcher's Block ends up being a 4 potency loss.

I'm not specifically confused about what you wrote. I'm more confused about how this blossomed into a discussion on how difficult enmity management is on WAR of all things.