Oh believe me I would have used that metric if there was one, but apparently there is no fecal storage tank as such; your poop is collected in a bag by the Universal Waste Management System or UWMS (which is what you call a space toilet when you're NASA and don't want to say "space toilet"), and those bags are stashed in a "removable fecal storage canister". Some of those canisters are returned to Earth "for evaluation" ("yup, it's poop"), but most are loaded onto a cargo ship that is then burned up on re-entry. Couldn't see any obvious telemetry for the UWMS' urine / feces separatator fan system kajigger either (the "Dual Fan Separator" + sort of gearbox, because apparently a space toilet needs a gearbox.)
