So after a discussion, we decided that lifting feats cannot be calculated using PE, which makes the Percy's feat invalid.
There's this feat of the sky falling onto Annabeth Chase though.
"Then the darkness above Luke began to crumble, like a cavern roof in an earthquake. Huge chunks of black rock began falling. Annabeth rushed in just as a crack appeared, and the whole ceiling dropped. She held it somehow[...]"
So unless I'm misunderstanding this, Luke was holding the sky, then Annabeth rushed in to support the weight.
There's no official height for Luke. I'm going to use the height of the actor in the movie version (fuck the Percy Jackson movies, but I have no other choice). Jake Abel is 1.83 m tall.
The sky is 5.3e+18 kg.
5.3e+18*1.83*9.81 = 9.514719e+19 Joules, around 22.74 gigatons, Island level
Atlas laughed. "How little you understand, young one. This is the point where the sky and the earth first met, where Ouranos and Gaia first brought forth their mighty children, the Titans. The sky still yearns to embrace the earth. Someone must hold it at bay, or else it would crush down upon this place, instantly flattening the mountain and everything within a hundred leagues. Once you have taken the burden, there is no escape." Atlas smiled. "Unless someone else takes it from you."
|~ The Titan's Curse|
There are 3 mountains inside this 100-league range, Tacocat calculated it to be Small Island level on Naruto Forums.
Mountain Tamalpais (the mountain they are all) is 747 m.
Atlas said it would instantly flatten the mountain. If we assume that means 1 second, then 747 m/s.
5.3e+18*747^2*0.5 = 1.47872385e+24 Joules, Large Country level