I choose three (or four) F's that are all things that I want to keep. They can be the same item; however, if they are different from each other the fuse goes faster. Because the E will be different, the fuse is already tuned up a bit, though, so I don't kill myself finding three different F's.
The closer the three F's are to each other, the more even the probability distribution between them for what you will get. The lower level items will have a greater probability of being the drop, which I find distasteful, so I aim for getting them as near each other as possible.
I always choose a cheap, unwanted, common-print E. The closer it is to the level of the F's, the better the odds of getting a legendary I want. If the E is too low, I have more odds of a "failed" legendary.
Ok, so to be concrete, here are a few that scored me some nice legendaries:
- Midas Gloves (F, 40), Balmung (F, 40), Plasmic Robe (F, 38), any level 35-40 common-print epic. Real good odds at an epic, and a 10-is percent chance of getting a legendary I want.
- 3x level 26 healer items (Singing Chakram, Elixir, Ygg Flute, Rejuv Tea), 1x epic Nordic Lute (because I am swimming in them)
- 3x Moon Voulge (F, 30), 1x Heaven's Will (E, 31)
I produce most of my F and E items by CF fusing, so the path to legendary is pretty quick compared to, oh, the way I did it a month ago. In fact, most of my cheap epics come from "failed" CF fuses, so very little goes to waste.