It's an indirect titration method similar to how the oxidation of iodine is used to detect chloramines. Her "evilness" is directly proportional to how greatly I fear for life & limb at any given moment; it's roughly a scale of 1-10 where 1 is purely happy lap-kitty and 10 is blockading myself in the bathroom holding my automatic Nerf gun at the ready.
Her residual evil-ality is roughly a 2-3. When she's taunted with a cat toy or throws herself against the window to "catch" a moth outside, the concentration of her evilness increases to a 4-5. At night, when she relentlessly attacks my feet under the comforter, it's nearing 7. Since there's no sunlight at night to expose her stealthy movements and keep her evilness in check, I sleep with one eye open and she's easily up to an 8/9. By morning, she's peacefully sleeping on her cat perch and her evil-ality has decreased back down to residual levels.