I'd second the rating of the BA Insulated Air Core pad being about 30. I took one out on a night that fell to around 20, and could feel the cold seeping through the pad around 35. (I'm not sure that cold seeps up, but since this isn't a physics lesson - yet - we'll let it stand as is.) If you're taking it into below-freezing, just buy an inexpensive closed cell pad and put it under the BA pad, and you should be fine. (I spent a winter night on packed snow with a Z-lite pad under a NeoAir, and stayed warm down to zero - in a twenty degree bag, with all my clothing on.)

Being from Ohio, I do hike in similar conditions to yours, and have spent some time along the Virginia portions of the AT. You can reliably subtract 10 degrees from the lowland forecasts to get an idea what the temperatures in the uplands will be - and allow another 5 ot 10, just for good measure. From the middle of April through the middle of September, a thirty degree bag is plenty (maybe too much in July and August), but you can always unzip it fully and use it like a blanket. The rest of the year, a 20 degree bag will usually be nice to have.