Evil Dog,
Try putting the shaft in a vise and count the vanes of the fletch with a needle, picking them apart every 5 or 6 vanes. This is the easy way to make a close and regular spacing.
To be honest, thread strength is less important if you are going to seal the shaftment with a verdigris based varnish, as was the usual practice with the English war shaft.
Judging by the Mary Rose shafts, where you can still see the traces of embedded thread in the shaftment coating, this should render the fletch binding almost indestructible.
I guess it also helps preserve the fletches on the stored shafts, the mediaeval equivalent of putting mothballs in your arrow storing tube.
Most of all it stops or cuts down the incidence of the thread being broken when you bury a shaft in the grass, which is how most of the bindings on my unsealed shaftments got broken, sooner or later.
Rod.