I use a powdered sugar (or confectioners sugar) sifter. It has a bowl-shaped screen the size of a fist at the end of a handle. This type of sifter has a finer mesh than say what you’d drain pasta with. I take the tobacco flour out of the coffee grinder and sift the fine stuff through the sifter. The rougher stuff left behind can get reground, but I usually toss it as a lot of that consists of stem material. After sifting, then I use the mortar and pestle. For best results (especially for really fine grinds), use only the lamina of the leaf (peel away the stems and leaf veins). This isn’t as practical for smaller-leaf varieties (like Oriental), though.