I’ve posted before about how to determine the “sett” of a yarn, wrap the yarn around 1″ of a ruler and count how many strands fit into that inch, 8, 11, 20 etc. If the yarn wraps around 10 times, you have 10wpi (wraps per inch) or epi (ends per inch). To determine from that what reed you need is simpler then I thought. If your yarn wraps 8 times, use an 8 dent reed, 10wpi = 10 dent reed and so on. What if you have a 20wpi yarn? then use a 10dpi reed and put two threads through each hole.
I have no idea why this has taken me so long to get my head around, I guess nothing explained it quite as simply as the All fiber yarns website.
Another good source of reference information on sleying the reed is from here where they have a “Weaving Sett Sleying Sequence Chart”. I believe this is telling me that if I have a yarn at 20wpi and I only have an 8dpi reed, then I sley the reed with a sequence of 2 threads in one dent, 3 in the next and repeat (2-3), if my sett was 22wpi, then my sleying sequence would be more like 2-3-3-3.
They also have a really good project worksheet which is great for keeping records, I had been hand writing mine but I’ll just print these out from now on I think.