The numbers being thrown around were derived from experiments in a closed system and it was 44lbs to reach lager temps. It would definitely take less ice to reach ale pitching temps AND the hot output water was not added back into the ice reservoir (which lines up with your findings). The OP is fleshing out ideas of a closed basin of ice as a heatsink which would act like the closed system (no method of removing the heat other than direct melting of ice). All in all, not a great idea.