Hi Darryl , what I did was measure the diameter of an old serving with calipers , that give me about .019 -.020 , then I ordered .021 .
I am using nocks from Bearpaw index clasic , the throat is quite tight and with the flemish string I can get a good fit .
I you are using Bohning nocks for example may be a better fit would be .025 .
I hope these can help you .
Answered on 9/20/2012 by Anonymous