Isn't SSO supposed to make twice the move of SPY? So if SPY fell to $45, a 50% fall, shouldn't SSO fall 100%?
no, because it is incremental. The SPY would have to somehow instantly fall 50% for the SSO to fall 100%.
You probably would get a closer-to-reality answer if you calculate it as two successive 25% losses. 0.75 x 0.75 = 0.5625 for a ~ 44% loss. Remember it is a goal, with no guarantees that it perfectly tracks the underlying stock or indices.