Good question for the PM folks. What does your equipment tell you? Power = force X velocity, right? So pedaling into a 30 mph wind, you maybe have a velocity of 10 mph. Force (RPE) is the same as doing about 30 but because of lower frictional losses you actually have a 40 mph headwind. So your PM should say about 1/3 of your usual still air power at 30, right? Something like that anyway. I'm making some assumptions with a large range of inaccuracy.