I have 55k HP, so the 8% Shake it Off is 4.4k. A 12% Shake it Off using Vengeance/RI/IB would be 6.6k. Lightning sprites hit me for ~1.4k, so Shake it Off should have broken on the 5th hit. Instead, it took 5 full hits and another 1k damage off the 6th, which is (1400*5)+1000=8000 damage shielded.

I have 66k HP with Thrill, and 12% of that is 7920, so Shake it Off includes Thrill's HP.