OK so using this, I just found a spot on the chart where the lines coincided to eliminate the need for extrapolation/guesswork (15,000 ft/27.5 miles). So, 27.5 statute miles is 145,200 feet and 145,200/15,000 = 9.68, so that's a ~9.7:1 glide ratio? So I guess I'll pad it for safety and put 9:1...