When to Count PPI
When I wish to weave at a particular ppi, do I count the threads under tension or with slack tension? I like to count when the tension is tight because it seems to me that when tension is released, the amount the web relaxes is not consistent, as when you have been weaving for several hours compared to, say, half an hour. When the tension is tight, I have to know how many fewer picks to weave: one less per inch, one less per 2 inches, etc. A good guess is okay for many items, but what if I want to achieve a perfectly balanced weave? What would be the best way to do this?
Your thinking is correct. After you release tension, depending on the length of time the tension is released, the threads continue to relax. A measurement taken with tension relaxed is not likely to be as consistent as one taken under tension (your weaving tension is usually very consistent). For balanced weaves, you are trying to weave a certain number of picks per inch to match the ends per inch. If you are using a temple (stretcher), the web is spread in the weft direction pretty close to the same way it is spread in the warp direction; so when tension is released, for example, a cloth with a warp and weft sett of 24 ends and pick per inch should relax equally in both directions. Therefore, working for 24 picks per inch measured under tension will give you a perfectly balanced cloth. If there is any variation, there might be slightly more take-up in the warp direction. You could guess about that, but I have never had a problem assuming that the relaxation will be equal for both warp and weft.
Different weave structures make a difference, of course. And, the denser the setts, the less change will happen after relaxation. If there is a great difference between warp and weft setts (warp rep or weft rep, for example), there will be a difference in relaxation between them, but in that case you won't be trying to match picks and ends per inch.
So, I'd do what you are doing and measure under tension, aiming for equal numbers of weft threads per inch as warp threads per inch with balanced weaves.