Xie Bai, or Lower White, is located on the San Jiao meridian (also called the Triple Burner or Triple Heater channel) between the 4th and 5th metacarpal bones, 1.5 cun proximal to the MCP joint.
Another way to word this is to say that the Xie Bai point is 1 cun proximal to Zhong Bai, 22.06.
eLotus, the educational wing of Evergreen Herbs, has an excellent video on locating and needling this point.