I only dub about two inches of thread at a time. Size 18 and smaller hooks I dub about a inch of thread at a time. If I need more dubbing I add a little to the thread when there is a little less than half inch of dubbed thread remaining. I twist some dubbing on the thread and push it up so that it touches the original dubbing. twist both ends together a little and you're good to go.

If I have too long a dubbed thread I remove dubbing when the body is the right lenght.

I very little dubbing. My dubbed thread is about the thickness of the hook I'm using. I think this the most important aspect of using dubbing, especially for dry flies.

Good luck!
Charlie