The proper pattern size depends on the usage size of the icon. I ’ve had best results with using an exact divider of the desired pixel size.
e.g. a 36px icon works fine with a 72px duotone pattern - a 144px pattern will also work, but this will markably enlarge the vector size. You might also encounter problems with path precision.
BTW The new importer supports transparency! You might give it a try as long as you don’t need simple font export.