What makes an invuln different from any other defensive cooldown is that you use only one cooldown to mitigate all incoming damage. Let's say that you've got a tankbuster and a follow-up cleave which are going to hit you for 1.3x your total health. Taking autos into account, you probably want to get to about 1.4x-1.5x eHP. You could stack two 20% DR cooldowns (1.56x) or perhaps bring out a 30% DR cooldown (1.42x). If it's a tankbuster that targets two tanks, you could be looking at burning through anywhere from 2-4 cooldowns from your shared pool. If you invuln, you can use a single cooldown and negate everything.

The added advantage is that negating a tankbuster in this way gives all of your other defensive abilities time to reset. So rationing cooldowns becomes a lot less important. That's why we have a surplus of cooldowns. You don't need to know the threshold. Just pop everything and invuln the next one. Your cooldowns will all be back up again when you need them.

I know that this is partially a fight design issue. You could add in more tankbusters, or, as I've suggested elsewhere, have more tankbusters that just ignore your invulns (like Ramuh's Shock Strike, back in the day). But it's much more easily solved by limiting invulns to being used only once or twice per fight (instead of three to four times).

Just as a side point, this issue of cheesing mechanics like Pantokrator is completely irrelevant now that Holmgang doesn't require a target and doesn't bind you. As if any of you would have ever agreed to sit in a corner and do zero dps just because your BRD or SCH wasn't willing to bait that mechanic in the first place. Yeah, right. Just bring it in line with the others. A three minute invulnerability move just doesn't make sense, either from a fight design or a balance perspective.