On bi-pins and rop bulbs you will notice the artifact shadows is caused by element wires, so light coming off the top, straight forward out the flashlight window is not shadowed. Therefore I frost only the lower two thirds or just from above the element level to the bottom of the envelope. Theoreticallly this will optimize the light out put but I wonder if the difference is measurable. I also find that a very light frosting goes a long way. Again the less frost the more light exits. Different glass, different bulbs frost at different rates. I work up a schedule for a bulb type for me 15 minutes at a time (frost, check it, frost again, etc.).