RE: Buying Field Points
You would need 21/64ths.
The first two numbers on an aluminum arrow denote the arrows outside diameter in 64ths of an inch and the last two numbers denote the arrows wall thickness in thousandths of an inch. So your arrow is 21/64 of an inch in diameter with walls that measure .013" or 13 thousandths of an inch.
Buying field points you do not need to know the wall thickness but that number comes into play if you ever need to replace an insert. Because the wall thickness would have an effect on the inside diameter of the arrow making it larger or smaller.
Hope this helped.