Engineered ventilation system - operable windows

There are two paths for showing ventilation compliance for an individual space or ventilation zone: Path 1 - Mechanical ventilation. In this case, you cannot take credit for opening windows. And this is where ASHRAE 62.1 mechanical ventilation rates apply. Path 2 - Natural ventilation. In this case for NC2009, operable windows are sufficient, subject to area and distance requirements for the windows. You could probably make a special circumstances or alternative compliance case for extending the distance from the window due to the assistance of the toilet exhaust fan, but ASHRAE 62.1 ventilation rate do not really apply to the natural ventilation case. I think you will get more traction by clearly identifying which path you are taking (either Mechanical Ventilation or Natural Ventilation) and sticking to that path. If you blend the criteria for the two paths together indiscriminately, it will be unlikely to be accepted. For example, you could say for Alternative Compliance that you are using the natural ventilation path, show that you have the required percentage of operable window area, and then introduce the concept of "fan-assisted" natural ventilation to justify any areas farther than the 25 feet criteria away from the window.