There is another concern about sett to consider: The Purpose. Do you want a firm or an open or a medium weight cloth? Figures A, B, and C show the same yarns, all used in a balanced plain weave (warps and wefts show equally). So, an open weave can be as balanced as a tightly woven fabric. What this means, is that there’s not one perfect sett for a given yarn and weave.
Allow for the purpose
You’ve seen that there can be a variety of setts for balanced plain weave. How do you select the sett for your purpose? You take Ashenhurst’s figure for the maximum sett and then take a percentage of it to suit your needs. Say, a yarn that doesn’t shrink much is to be used for upholstery in a firm, plain weave that will be dry cleaned (no shrinkage). If the maximum sett for the yarn is 30, then 90% of that would be the ballpark figure to try for upholstery fabric (90% times 30 = 27 epi). Then say you wanted the same yarn for pillows (not such a firm fabric) you might use 80% of the maximum sett number for your pillow sett (80% times 30 = 24 epi). What if you wanted a shawlÑloosely woven–to throw over the chair? Then take 50%-60% of the maximum sett (50% times 30=15 epi). Can you see how helpful his formula becomes? When you calculate the diameters and then the maximum sett for the weave structure and yarn, you can take percentages of the number to allow for what you want to weave. Woolen fabric is woven loosely with the intention that it will be shrunk in the finishing process and end up a firmer fabric. 65% is a good number to start with when determining the sett for this type of yarn and fabric.
Here are the percentages of the maximum sett for some types of cloth. You can go higher or lower as you determine what your yarn or purpose requires.
Again, the reason for wanting to know a maximum sett is so that you can take percentages of it to allow for different yarns and types of fabrics. These are numbers generally used for balanced setts, where the warp and weft show equally. Open up the sett (so that the wefts can pack down between the widely spaced warps) for more weft predominance, and make it closer (more warps per inch) for more warp predominance.
More information is in Weaving for Beginners and Book #`1: Winding a Warp & Using a Paddle.