Drug-induced long QT — which medications are highest risk and how do you monitor?

48F with schizophrenia on haloperidol, recently started azithromycin for community-acquired pneumonia. Routine ECG shows QTc 520ms (up from baseline 440ms). She is also on ondansetron for nausea. At what QTc threshold do we stop the offending drug? What electrolyte corrections are needed? When is the risk of Torsades de Pointes highest?