offset calculation using uncalibrated WebSDRs
Only few web receivers concepts (such as Perseus) offer spectrum resulution of 100 Millihertz RBW and less. But if you use LSB/USB/CW modes you can analyze the audio stream. I prefer the Spectrum Lab software, feeding it with the Stereomix of the sound card. Even though the source is compressed, measurements can be extremely precise unless resampling issues occur. What you need is two reference frequencies with a defined deviation or a defined frequency of the carrier's audio (in CW mode). Keep in mind that CW is like LSB.
Table based on 2 references:
Deviation is of the kind m*x+b. This situation occurs in a previously calibrated Perseus recording.