Why it’s so wonderful to weave with a slightly closer warp sett:

1. The edges of the cloth (selvedges) don’t draw in as much, so the selvedges don’t break. The extra warps hold out the warp width.
2. You don’t have to make sure the weft is put in very loosely. The natural diagonal the shuttle makes when you throw it through, from the last row of weaving to the beater, is usually enough slack for the weft’s pathway. (If the weft is put in without any slack, it will pull in the edges of your cloth.)
3. There is less trouble with warp breakage. Fragile warp threads can be used because there are more of them to pull their weight on the job.
4. There are fewer wefts (picks) per inch so the weaving goes faster. Now, don’t those reasons sound enticing?

In general, whenever I’m debating between two numbers for a sett, I’ll choose the denser number for the above reasons. That means, if I were debating between 6 and 8 epi, I’d tend to choose the higher number of 8. If I’m weaving an open shawl, however, I want it more open than the 80% number, for sure. Of course, my sample will be the ultimate test.

When you don’t want to use the 80% figure

You need to account for the texture of the thread when you are considering how low a sett to choose (minimum sett). Hairy yarns wouldn’t be good for the 80%. Woolens are meant to shrink, so the 80% wouldn’t work for them either. Smooth, slippery silk or rayon threads need to be sett closer than 80% (more warps per inch) than woolens because the slippery yarns will create unwanted spaces after you wash your cloth (called slippage), spaces that hairy yarns would fill up.