When I have bought (gasp) comforters for our bed I ALWAYS buy a king size. It lays on the bed nicer and less chance of us fighting for the covers! If I were you I would just measure the bed to the spots I wanted the quilt to lay and then just make it according to the measurements!
I never follow directions for making quilts a certain size. My baby quilts are normally 48x60 and my lap quilts are normally 60x72 unless I am making it for my husband and then its a "king size" blanket!