In The Case of the Mandarin's Treasure Chest, Pao-Tan was the ancestral home of Wung Ling. Biggles accompanied him there in order to recover a chest of valuable Chinese cultural treasures which he and his father had buried when that part of the country fell to communist forces during the Chinese Civil War. According to Wung Ling, Pao-Tan was in the province of Kweichow (today Guizhou) not far from the place where the Burma Road makes a sudden turn to the north near Chungking (today Chongjing). In was in the valley of the Shangpo River and most of the terrain had been washed flat by the river when in flood. It was a undulating landscape where most of the trees had been felled to make way for the cultivation of millet and barley.
Pao-Tan and Shangpo are fictional locations. As can be seen from the map below, the place where the Burma Road turns abruptly north is the city of Kweiyang (today Guiyang). Biggles followed the Burma Road and a quarter of an hour after the point where the road turned north, he throttled back and descended to land. This would make Pao-Tan about 40 miles north of Kweiyang.
During the Second World War, the China terminal for the Burma Road was Kunming, the city at the bottom left of the map. Sources differ on whether the stretch of mountain road after Kunming is part of the Burma Road, but certainly in the map which was published by the U.S. Army in 1941, the label Burma China Road is used for the stretch after Kunming (see just above the first blue arrow on the left).
Map of the Burma Road which is the thin red line across the map. Kunming is to the bottom left. Chungking is top right. The blue arrows indicate Biggles's approximate flight route. If Pao-tan is about 40 miles north of Kweiyang, it would be around the tip of the blue arrow to the right. Johns was correct that the landscape changed from rugged terrain to more open country once the Burma Road turned north.