In Response to comment: https://www.intuitibits.com/2016/03/23/dbm-to-percent-conversion/#comment-9 regarding the quadratic formula used by WiFi Explorer (based on ipw2200 driver logic) for which a dBm to Percent table is provided.
Some quick PHP based on the ipw2200 code and a table generating function:
function signal_quality_perc_quad($rssi, $perfect_rssi=-20, $worst_rssi=-85) {
$nominal_rssi=($perfect_rssi – $worst_rssi);