I think you misunderstand the purpose of the Mass Air Flow (MAF) sensor (and whoever wrote what you read doesn't understand, as well). The purpose of the MAF is to read the amount of incoming air. As the incoming air flows over the heated sensor, it cools the sensor which regulates the amount of electricity which can flow through it. It is in no way associated with the RPM, nor does it read the RPM of the engine. The only way that an increase of RPM would affect the MAF output is because inherently there is more air going through the MAF when the RPM's increase. This is not a direct reading. If it were as you are suggesting, there wouldn't be a need for a Throttle Position Sensor (TPS).
Also, the ECU doesn't use the voltage of the MAF to determine the idle RPM. The ECU has a target idle RPM it tries to maintain by adjusting the timing, plotting the fuel injector pulse width against the fuel map, and adjusting the Idle Air Control (IAC) valve.
As far as any correlation formula or graph, there isn't going to be a set one. And Ford is going to maintain these as proprietary data. Each vehicle model is going to have a different one. You would have to take tuner software, read the tune for the specific vehicle you want to examine, then create the correlation from the data you extracted. Let's just call it reverse engineering. It would be a generous tuner who would put this information out there for common consumption.