In a recent Ask Madelyn, you stated that "On countermarch looms, the closer the shafts are to the fell, the bigger the potential shed." This seems counterintuitive. My experience has been that the height of the shed decreases on my countermarch Toika loom as I weave and the fell approaches the shafts; so I would think that moving the shafts farther back from the fell line (rather than closer) would increase the height of the shed.
That is true, but for a different reason (this seems somehow related to the law of diminishing returns). There is a point where the shafts can become too close to the fell and the potential shed opening therefore becomes constrained. For example, if the fell is an inch or so away from where the shafts are moving apart they can't form an opening of many inches. Tension on the warp would prevent them.
But think about the shafts themselves in relation to each other. The shafts that are farther away from the fell have to make a larger opening for their warp threads to make the same opening as the threads on the closer shafts. If you tie up a multishaft countermarch loom (eight shafts or more), you therefore have to make the back shafts move more than the front shafts. (This also means that the treadles and lamms have to move more, which limits the number of shafts a typical countermarch loom can operate effectively.) For four or six shafts, this difference is not very significant.
For unrelated reasons (consistency of beat), it is good to advance the warp often so that it never gets so close to the shafts that the shed starts to become constrained.