No, don't ease the right angle into a straight line. The 8 cast off stitches (about an inch?) should be sewn along the sides of the sleeve. The straight top of the sleeve is sewn along the rest of the front and back edge. In other words, if you stitch the shoulder seams and then lay the front and back down flat, the sleeve should be insterted into the armhole shape, with the right angled corner of the sleeve fitting into the right angled corner of the front/back. I hope that makes sense!