I don't fish pa steelhead anymore but it used to be when the brandy run gauge was at 8-10 cfs and dropping. This correlated to good water on the area streams, assuming all of the tribs received around the same amount of water.
One thing to remember for you out of towners, those streams drop FAST. As in, 3' over the banks flooding people's yards today and Thursday afternoon it's low and clear. That's not much of an exaggeration.