I'm embarking on my first, real-live "spin a specific yarn for a specific pattern" odyssey, using the two pounds of dyed purple "wool top" mentioned in this post, spinning a worsted weight yarn for this lovely pattern.
Here's what I need: ~1300 yds of worsted weight yarn, with a grist of 840 yds/pound. The good news is that since I have two pounds of fiber, I've got a bunch to sample with before I run up against the 1.5 pounds I'll need for actual yarn for the sweater.
Other things I "need": a yarn structure that will work well with garter stitch and the fancy stitch pattern in the yoke. To me this is crying out for at least a 3-ply yarn, just because lots of plies make for a rounder yarn that rocks the hell out of the garter stitch.
I also "need" a way to spin this that won't A) take forever and ever, amen and B) will give me a lofty, puffy, snuggly yarn that will result in a lovely snuggly sweater.
Sampling Plan A: the 3-ply versus the 4 strand cabled yarn. I took two 1 oz chunks of fiber, and spun them up using my version of long draw, from the fold. For one aliquot of fiber (that's not the right word and it sounds really funny to use it in this context, but it will work), I split it in three equal chunks, spun each chunk on to a separate bobbin at 7.5:1, and did a true 3-ply.
The second fiber aliquot got split into 4 chunks, spun the same way. Two sets of two bobbins were plied together at 9.25:1, aiming for an overplied 2-ply. Then those two bobbins of 2-ply were plied back on each other at 7.5:1.
I checked the wpi of the two sample skeins before finishing: the 3-ply was about 14 wpi, and the cabled yarn was about 12 wpi. I was aiming for about 10 wpi in the finished yarn. Since both yarns were a bit on the fine side, I did a fulling wash: yarn was dunked in hot water with Soak with a bit of agitation, then put in cold water and agitated again. Repeated the hot/cold switch until the strands in the skeins started to stick together a bit, then squeezed out the excess water, thwacked against the side of the house, and hung to dry.
Here they are post-wash, 4 strand cable on the left, 3-ply on the right. I ended up with ~30 yds of the cabled yarn, and 60 yds of the 3-ply. As expected, the 3-ply ended up a bit on the light side, coming in with a grist of 1066 yds/pound. The cabled yarn was much closer - 800 yds/pound.
Now it's time to knit up some (gasp!) swatches. Thankfully, just in garter stitch. I also need to consult The Grist Bible and perhaps some of these technically inclined folks to see how close my grist needs to be for this scenario. Plan B, if these really don't work out, will be a true 4-ply with lots of plying twist, and maybe a 5-ply (!).