Power lunch ladies Maria Bartiromo, Marjorie Gubelmann, Jane Holzer, Pat Cleveland, Janna Bullock and Fern Mallis got a lesson in correcting “BlackBerry posture” at a gathering for Canyon Ranch spa in an 18th Street loft space the other day. The company’s exercise physiologist, Mike Siemens, taught the women to sit up straight to correct the hunching over that comes from constantly looking down at one’s handheld device. They ate a four-course meal that included chocolate cake yet only amounted to 700 calories.