From what I read, it will take 15X1, HAX1, 130/705H which are the same that the Brother uses. If you are using a pattern, I would suggest that you cut larger and possibly taking a larger seam allowance (for flannel). I don't know if you can change the stitch length on your Grain, but it has been suggested to use a shorter stitch length with flannel.
Janey - Neat people never make the exciting discoveries I do.