Is it before or after you check which damage roll was greater?
For example, a level 5 summoner & eidolon get damaged with the same effect. The summoner saves for 18 damage and the eidolon saves for 17 damage (it has some resistance from reinforce eidolon). 18 damage is taken as the greater effect. Using Reactive Dismissal will reduce the damage by 10.
Would it reduce the specific 17 damage the eidolon saved to 7, but since the summoner still saved for 18 damage, they still take 18 damage? OR Would the reduction apply to the overall greater damage roll between the two, in this case 18, so they take 8 damage?
I have another question/scenario that will likely be answered by that already. I'll put it in spoilers just in case.
For example, the summoner saves 80 damage and the eidolon saves for 20 damage from the same effect. 80 damage is taken as the greater effect and is enough to KO. Is it NOT possible to use the summoner precaution reaction because the eidolon only saved for 20 damage and would not KO, OR is it possible because the 80 damage is the actual damage taken even if the summoner is the one that saved for it?
Is it before or after you check which damage roll was greater?
Before. It reduces the damage the Eidolon takes, like Resistances and the like. Then you choose which one you take from the Eidolon and the Summoner damage.
I can see a strict RAW case trying to squeeze the damage reduction after the Eidolon damage has been chosen but it'd clearly go against RAI (and common sense, for what it's worth).Similarly with Summoner's Precaution, it cancels the Eidolon damage so you take the Summoner's damage instead.
I'm not sure if 'before' or 'after' rolling the damage is really the right question to be asking.
You can only stop the Eidolon from taking damage. If you are both taking damage you could use the spell's reaction to unmanifest your Eidolon when the Eidolon would take damage. That wouldn't prevent the Summoner from taking the damage if they were caught in the same AoE.
So if you want to say that you decide how much damage each takes first and then choose to unmanifest the Eidolon, I am not seeing how that would be a benefit. The Summoner is still going to take their own share of the AoE damage then since the Eidolon is no longer manifested.
So if the Summoner was taking the higher amount of damage, then they are still taking that amount after unmanifesting the Eidolon. If the Eidolon was taking the higher amount of damage, then the Summoner would only be taking their own amount of damage - but that damage wouldn't be subject to any damage reduction of the Eidolon.
What you probably want instead is Protective Bond so that if the Eidolon saves against the spell effect then the pair take the lower amount of damage instead of the higher amount.
And since all of these are reactions - specifically reactions with a trigger of the Eidolon taking damage - then you could only use one of them.
Specific Overrides General...
Specific doesn't define general.
Thanks for clearing that up, everyone. Good reminder for Protective Bond too. I forgot about that feat.
Reactive Dismissal triggers when the damage would be inflicted. That necessitates it coming at the end of the calculation, because at any time you're still calculating damage you aren't actually taking it, you don't even know if you're taking any until you're done with all the calculations.
Further, there's no scenario in which 'only' the eidolon takes damage, or 'both' of you take damage. If you're both caught in an AoE you calculate damage twice, but you only ever take damage once, and the damage goes to your collective pool of hit points, because that's the only HP you or your eidolon have.
So Reactive Dismissal would apply to whatever damage you end up taking and reduce it by 10... because that's just what the ability does.