RST report with no S-meter

Some hams have difficulty giving an RST report (or RS report for SSB) when they are using a rig with no S-meter. The S-meter is useful for giving a comparative reading between the noise floor (RF level when the station you are trying to hear is not transmitting), and the signal strength of the station you are attempting to hear. If the S-meter were properly calibrated (which it usually isn't), then each S-unit change would indicate a 6db change in signal strength. This is why you must quadruple your TX power if you want the other station to hear you 1 S-unit stronger. So if you are transmitting at 5W output power, then increasing to 20W would get you 1 S-unit, and 80W would get you 2 S-units stronger. This is one reason why it is best to improve your antenna system rather than increase your TX power a few watts.
However the S-meter should not be used to create an RS/RST report, as Readability/Strength is a "subjective" report based on how you "hear" the other station.

A 5/9 report indicates the other station is perfectly readable and is very strong. But lets take a couple of examples:
If RF conditions are very favorable with the noise floor showing an S0 on an S-meter, and the signal you are copying is an S2 to S3 on the S-meter; then he could be given a report of 5/8 or even 5/9 since his signal is very strong compared to the noise floor.
On the other hand, if the noise floor were an S8 or higher on an S-meter and the signal you are copying is an S9; then his report might be a 4/4 since he is not much above the noise floor. Static bursts might prevent you from hearing every word which could result in you giving a 3/3 or 3/4 report.
You should keep a copy of the RS or RST report chart near your rig so you can give valid reports to the other station - of course in a contest, everyone is a 5/9 no matter how many repeats are needed to get his info correctly !?!