In-situ slack-time monitoring may be used to enable ambitious power management policies under circuit wear-out and dynamic temperature and supply voltage variations. Given a limited hardware budget, it becomes crucial to be able to select the most appropriate places for in-situ slack-time monitoring. Here, two metrics are proposed to guide the selection of a set of flip-flops (FFs) for in-situ slack-time monitoring. The goal of these metrics is to maximize the ratio of clock cycles with at least one monitor activated and the number of activated monitors per clock cycle. The activation probability of a monitor is evaluated with the help of timing simulations as the probability that signals are propagated along the monitored timing-critical paths. It is shown that in-situ slack-time monitors with detection windows correlated to the minimum slack-time of the monitored timing-critical paths can provide better results than similar monitors with a constant detection window for the same impact on the circuit latency.