The general rule is that the fell line should be contained within the 'sweet spot' - approximately 2" right between the breast beam and the reed.
There are times, however, when it helps to adjust where the fell line will actually be.
In the above photo, the fell is much closer to the breast beam than I would ordinarily position it. The entire sweet spot needs to shift closer to the breast beam in my experience.
Why?
Well, rayon chenille is textured. The fell doesn't open cleanly and I find that by moving the sweet spot closer to the breast beam the shed opens 'better' so that the weft can be beaten into place.
I know that a lot of people advise not using a great deal of tension with rayon chenille, but I find that by using a somewhat higher tension on the warp and moving the fell, I can get a decent shed, seat the weft around the selvedge end and beat the weft into place.
Now this might not work on a jack/rising shed loom. A counter balanced loom forms the shed by sinking the shafts you want down while simultaneously raising the rest. This makes a shed that has equal tension on both sets of ends - those down and those up.
There are other instances when moving the shed seems to help - any yarn that is hairy; any yarn that is textured; any warp that is extremely dense.
It is also possible to adjust shed geometry by inserting lease sticks and shortening the distance between the fell and the back beam. Another way to adjust shed geometry is to increase the height of the back beam. This seems to particularly help on a rising shed loom when weaving linen, especially if the overall length from breast to back beam is short.
Like any other tool, there are times when you might consider adjusting your equipment, or adjusting what you usually do with it.
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