Scoring only the largest amount in the window catches 95.2% of the suspicious cases. It also alerts on 16.7% of the benign ones, because each of those is a one-off family purchase that the history explains.
95.2%catches 95.2% of suspicious cases with the amount scenario at 0.50, interval 84% to 99%, on 42 cases
16.7%false alerts 16.7% at 0.50, interval 8% to 31%, on 42 cases, all benign