Ruling Donphan Expert Belt

[CyndaquilMaster]

OK, so this came up at States a while back and I was wondering about it. So I had a Donphan active against another Donphan that had an Expert Belt. My Donphan had 40 damage on it. His Donphan uses Earthquake for 80 with the belt. I thought that Donphan was still alive because of Exoskeleton. He said that Expert Belt Knocked Out my Donphan before Exoskeleton was applied. I argued that you check for Knock outs after all effects. We asked a judge and he said that because Exoskeleton takes effect after weakness and resistance this happens.
40 (Damage on my Donphan)
80 (Damage done to me by Donphan's earthquake)
120 (Combined Damage on Donphan)
Knock Out
-20 (Exoskeleton takes place after the knock out happens)

Now should this happen in the order of events previously shown?

 

[RB Golbat]

You only check for knockouts after all additions/subtractions to damage happen. No damage/damage counters are put on a pokemon until all applicable Poke Powers/Bodies, Weakness Resistances, other effects have been calculated.

Donphan should have survived.