This is how I make these hems. Say I want a 1/4" hem. Accurately stitch at 1/2" along the edge (one layer). Spritz lightly with starch and then fold the outer edge in half to meet the stitch line. Again carefully press easing in any fullness. The stitch line will keep the curved bias edge from stretching. The edge should lay flat with no "pointy things" to the o/s edge. Stitch carefully as you normally would. Press edge again. -- For a narrower hem, you could reduce the 1/2" to 3/8", press in half, then stitch.
Let me hear how if this works for you. GOOD LUCK!!