Best Reed Size for 30 epi
In general, you want as few ends in each dent as your collection of reeds will allow. So, following that principle, for 30 ends per inch, you’d use your 15-dent reed. This principle is particularly important if you warp front to back, i.e., sleying the reed and then threading by taking the ends as they come in each dent. For most yarns, this works well with 2 ends in a dent. Twisting between reed and heddles can be a problem with 3 or more ends per dent.
More ends in a dent can also result in reed marks in the cloth. Most reed marks come out with washing, but not all (and not with all yarns). It’s also important to distribute the ends evenly if you have more than 2 in a dent. Let’s say you used your 8-dent reed to get 30 epi by sleying 4-4-4-3. With some yarns, the dents with 3 in them might show as streaks of slightly different density along the cloth, especially if the warp and weft contrast in value.
A separate issue is one that you bring up. If the yarn is especially fragile or slubby or the warp particularly dense, reed abrasion can be a problem. In that case, you’d want to use a more open reed (still keeping all of the above in mind).
Thanks for writing!