The sweet spot for RSSI between neighboring nodes is between -55 and -65. The closer to zero, the stronger the signal. So an RSSI of -55 is a stronger signal than -70.
You can find the RSSI between your APs by clicking on them in the map view and going over to Neighbors tab.
You should also try to limit the number of neighbors to about five or less. Any more than five neighbors and you could experience performance problems due to signal interference.
See also: RSSI dBm ratings