Arg, I just figured it out guys. As it turns out, I'm a jackass. I was figuring my batch size at 5 gallons, which is what goes into the fermenter. I forgot that I was leaving about 1/2 gallon in the kettle with the trub. 6.5*46=299 / 5.5*50=275, that's a heck of a lot closer than 5*50=250...