the "right number" is strongly dependent on what the consequences of false positives are. If you're comfortable pulling numbers from thin air, 0.5% of these guided patrols lead to prevented crimes, but 10% lead to arrests of simply suspicious looking people (suspicious on grounds of being around where crime is predicted) and then while they're being detained are late for a job and get fired. Is 20 people getting fired worth preventing one crime? Say, a catalytic converter being stolen, since "crime" is very bad, not, say, murder particularly.