With aluminum arrows, the first two numbers are arrow's diameter in 64th's of an inch, the second two are the wall thickness in thousandth's of an inch. For instance, a 2020 is 20/64 (5/16) of an inch in diameter, and the shaft walls are 20/1,000 of an inch thick.

The Easton chart usually gives a spine way high for trad bows. Try the one on bowjackson.com or arrowsbykelly.com. May not be dead-on for you, but will get you in the ball park.

Chad