A simple, but quite useful calculator, to determine how many times further your RF link can go given a number of dB increase (or decrease) in power.
As an example, your 500mW (27dB) transmitter is putting out 1dB less than it should (26dBm, or 400mW), what effect does that have on range?
Answer: 0.89 times, or 90% of the specified range
As another example, what kind of range increase should you expect if you increase your antenna gain by 11dB (e.g. upgrading from a SpiroNET Omni to a SpiroNET 13dBi patch antenna)?
Answer: 3.55 times, so if your range was 1km before, it will now be closer 3.55km