Aircraft structures are designed to withstand ultimate load associated with the maximum expected maneuver during the entire life of the platform. As a precaution, they are mandated to fly below a prescribed, lower load limit. In the presence of damage, the structural load carrying capabilities are reduced as the damage grows, posing a safety issue. A primary goal of a real-time Structural Health Monitoring (SHM) system is to detect a damage, under normal operating scenario, well before the damage reduces the structural integrity, in terms of hindered ultimate load carrying capabilities and, more strictly, limit load carrying capabilities. An SHM design tool is proposed for the quantitative assessment of the margin between a robust detection of a damage at a known load and the ultimate load characteristics of the structure, as a function of platform loading and damage size. When a digital twin is available and the aircraft loading spectra are known, it is possible to simulate the dependence of properly processed readings of a given SHM sensor-net as a function of aircraft loads and damage size. These data can then be arranged into an SHM design map that characterizes the SHM detection threshold in terms of aircraft loading envelope, damage size and structural load carrying capabilities. In the present work the concept of such an SHM design map is presented and applied to a digital twin, representing a damaged wing spar cap. It is also experimentally tested using a a fiber-optic-based SHM system, which monitors a spar-to-skin de-bond.