Never heard of a number before an I had 5 premature babies and one full term!
I had the Serfactant dhot with two of my babies and it worked good but the lungs are not the only thing underdeveloped before 37-38 weeks.
The amnioctic fliud can be tested by amnio and it will show if there is the hormone in it, it shows if the baby is breathing or not as it will be present in the amniotic fluid. I have no idea though about a number! Sorry
I cant answer your question, but can share an experience. I had my son at 32 weeks, his lungs werent fully developed and ended up having a shot survactant (its something the lungs produce to help them from collapsing) sorry sp. It helped with him and he was breathing well without equipment a few days later.