it's gonna be hit and miss to figure out the right serving fiber diameter if ya don't have access to a multiple of serving diameters.
450 is a large diameter string fiber, so you might do with BCY #2D spectra serving, it's .016".
with the dyneema'02 i use, i like BCY Halo in .019". now, this is for a LOOSE fitting nock that won't stay on the string by itself, the way i like it for a spit finger grip.
if yer string grip is 3 fingers under, you'd hafta build up the nock area with floss to get that very soft snap fit.