How does RSSI (dBm) relate to signal quality (percent) ?
Depending on your OS and application, WiFi signal strength is represented either as quality in percentage, or an RSSI value in dBm, i.e. -70db. RSSI is usually expressed in decibels from 0 (zero) to -120db and the closer it is to zero, the stronger the signal is. RSSI level less than -80db may not be usable, depending on noise.
While there is no simple precise solution that is used universally, we will try to explain the approximate correlation between signal (RSSI) and quality (percentage).
db >= -50 db = 100% quality
db <= -100 db = 0% quality
For RSSI signal between -50db and -100db,
quality ~= 2* (db + 100)
RSSI ~= (percentage / 2) - 100
High quality: 90% ~= -55db
Medium quality: 50% ~= -75db
Low quality: 30% ~= -85db
Unusable quality: 8% ~= -96db
See also: How to read RSSI/signal and SNR/noise ratings ?
Note: Most networks can work well with signal over -80db.