High school lasts 4 years. Some people play for all 4 years but if the average is 2 years you’re looking at 0.5 million people playing for the first time each year.

Most players aren’t drafted. The average NFL career lasts 3.3 years. 2400 / 3.3 = 727 new players per year.

500,000 / 727 would be 1 in 687 or 0.15%.