If you're measuring SSID's in dBm, chances are you're on-the-go. And what better way to measure SSID's than using an iPad?
Oscium's WiPry-Pro is the capable of measuring SSID's in dBm (or mW if that suits your fancy). See RSSI column for the exact value in dBm. SSID Hebel_2.4 has a value of -55 dBm: