I'm getting ready to weave a project from Handwoven, "Happy Towels, " March/April 2011, pages 52-53. The instructions allow for 31" for loom waste and 13" for take-up, and the 5-yd warp provides four towels (with a woven length of 34" each). I want to weave eight towels, and I'm not sure how to figure out how long the warp should be.
Your question is a good one and is connected to an issue that has always been very tricky in project instruction accuracy. In theory, we want to know exactly how much warp length is needed for a given woven length of fabric. In our project instructions, we usually give a woven length "measured under tension on the loom." When you remove the item from the loom, that length will decrease as the fabric relaxes. Even on the loom, however, the warp threads in the woven fabric are no longer straight–they bend over and under the weft. The difference between the relaxed fabric and the actual warp length used to make it is "take-up." The difference between the relaxed length and wet-finished length is "shrinkage."
We are not usually able to determine take-up accurately. To do so would require that the weaver measure the actual length of warp used to weave each item, a measurement that could best be determined by measuring the length of warp NOT used and subtracting that from the original warp length (and, unfortunately, with the variation in warping boards, the original warp length is not likely to be an extremely accurate measurement). Another issue is that weavers don't always measure the amount of warp they use up when they are tying onto the front apron rod and include that with the measurement for loom waste.
All this is to say that giving an accurate number of inches to allow for take-up is next to impossible. A number to allow for shrinkage is easier to determine, since measuring the item before and after washing can be done with accuracy.
In project instructions, we do the best we can to make sure that warp length is more than sufficient for the items in the project with a generous allowance for loom waste (at least 27", but note that this will be different for different looms and tie-on methods). For this project, if woven length measured under tension on the loom is 34" per towel and the interlacement is plain weave, I'd allow about ten percent for take-up (probably more than really required)—about 38" warp length per towel.
But even more, I'd recommend always putting on a longer warp than is required by the project. You'll come up with ideas for varying the design and colors as you weave and be sorry if you run out of warp before you try them. I usually put on twice as long a warp as I think I'll need.