Received signal strength (RSSI) of the last message received by a device should not be evaluated alone when determining if the network is healthy because it does not specify which device sent the last message. When data is harvested, the last message received could be a device on the complete opposite side of the network.
Instead, RSSI should be used for the following reasons:
a secondary debugging mechanism when a device is communicating poorly
when a device is showing a lot of harvest failures or often showing up as yellow in the app (no data for 20 minutes)
To determine the health of a network, look at the device summary in the app terminal. All or most of the devices should be green; a few yellows are okay, but should not be the norm.