As covered in What should my RSSI levels look like?, the recommended minimum RSSI rating is -65 dBm for repeaters and clients to get the most reliable and strongest connections.
What may not be obvious is the exponential nature of a dBm unit vs the linear nature of standard numbers.
For example; the difference between the numbers 65 and 71 is only 6. Not even a 10% difference between the two numbers.
The difference between -65 dBm and -71 dBm, however, is vast. A -71 dBm connection is only 25% of the received signal strength of a -65 dBm connection. A -65 dBm connection is 75% stronger than -71 dBm.
The difference between a -75 dBm connection would only be 10% of the received signal strength of a -65 dBm connection. A -65 dBm connection is 90% stronger than -75 dBm.
When you're looking at the RSSI ratings, here's a quick dBm reference:
The rule of 3's: For every +/- 3 dBm difference, multiply or divide the strength of the received signal by 2
The rule of 10's: For every +/- 10 dBm difference, multiply or divide the strength of the received signal by 10
Determining the exact percentage change of dBm outside of changes in the amounts 3, 6, 9, or 10 would require a calculator and more advanced formulas, but this should give a rough idea of the percentage changes to the signal strength your devices are seeing as the dBm based RSSI values change.