I'm also thinking you are talking about a mid or long arm? Dunster has your answer for this. But know that there are many different poly threads--i.e. 50# So-fine is much thinner than #30 So-Fine, and the trilobals like Glide are thicker than #50 So Fine, Bottomline,etc. And thus would use a different size needled--I use an 18 for Glide, but a 16 for #50 So-fine. Hope that helps.