I have made my own strings for a couple of years (B-50 only so far).  For serving, I have been using #4 nylon.  It's okay, but now I want to upgrade to something more durable for a serving.  I know I could go with halo, but it's pretty expensive.  

If I did buy halo or some fast flight type material, I still don't know what diameter would work best for a 14 strand B-50 string to fit Bohning Classic index nocks.  Any help out there?

I suppose you are talking endless string here?!

I employed my string diameter formula - feed with your datas - and would advise any .020" or .021" serving for you.
String diamter would be 3.05mm or 3.11mm and 11/32" Bohning classic nocks are 3.05mm wide.

I use #4 only for loop servings and like Monofil-Nylon #15, #18, #21 for the center serving.

[25.4mm = 1"]

Sorry--Flemish string and the center serving.

Scott with a 14 strand B50 .022 brownell Diamondback serving works great with Classics.

Thanks, Falk and LeRoy.

I build primarily dacron strings for myself and have been using Halo.  I really like this stuff, except for the price.  Still, you cannot wear this stuff out, even if your arm slaps the heck out of the string on every shot!

I use .018 diameter on my 14 strand strings.

Brian