According to https://en.wikipedia.org/wiki/Lung https://en.wikipedia.org/wiki/Lung, Estimates of the total surface area of lungs vary from 50 to 75 square metre

According to https://en.wikipedia.org/wiki/Lung https://en.wikipedia.org/wiki/Lung, Estimates of the total surface area of lungs vary from 50 to 75 square metres (540 to 810 sq ft);[28][29] roughly the same area as one side of a tennis court. They then offer two references for this. So you're off by a factor of 2 but are basically correct.