Here's what I'm doing: I put 2 pins in my spinning apron (an old towel) that are 2" apart, exactly. I set up my wheel on the 14:1 whorl, and then as I spin, I carefully draft out 2" of roving between the pins for one treadle, and move my hand back to the roving during the 2nd treadle. So: 2 treadles for every 2" drafted. This is a classic short forward draw, to produce worsted singles. At this whorl ratio, that means I'm getting 14 tpi (twists per inch). If I chose my 10:1 whorl, I'd get 10 tpi in this way. As long as I am careful about drafting between the pins every time, and holding to the rhythm, I'll get consistent yarn. I'm aiming for a single that's the thickness of buttonhole thread.
[use pins to control how much I draft per treadle]
I'm finding that 2" is a comfortable amount to draft out of this fiber (Suffolk). I tried doing 3" (that way, I'd get 28 twists over 3" or a tpi of about 9), but this was hard to keep consistent as the natural fiber length of this Suffolk doesn't lend itself to larger bites of roving. I'm guessing that this is a variable you have to play with, depending on the roving you are spinning. I also find myself lapsing into long-draw as my attention wanders, but then I have to snap myself back into the forward draw to really fixate on the twist. Did I mention discipline already? This is slow, tedious work, so you need to get into the Zen of it.
For plying - I'm doing a 4-ply cable here - I use the same pins, overplying by a factor of 2. So: 4 treadles for these 2". I'm using the pins to keep myself on track! This is even slower than the spinning! Then on the final ply, the yarn just snaps into place without me really concentrating very much.
A small sample so far looks like this - quite tight, and the yarn with less spring than I've been getting with my usual long-draw technique from downs breeds (I think the cable-plying and the tight twist "eat up" the inherent springiness of the fiber). The yarn is very dense; although this test sample is quite fine wpi-wise, the grist is lower than I expected. I don't think I'd try 14tpi with a longwool, and certainly not a cable-ply at this level of twist... The yarn is harder than I usually produce, and I'm wondering if it isn't actually too harsh...I will have to knit it up to see.
[sampling: 18-20 WPI and a grist of 2600m/kg]
A by-product of the back-to-basics exercise is that I'm developing a sense of what high-tpi singles feel like. As I get better at this, hopefully I'll be able to advance to spinning longdraw, using higher-speed ratios - a faster way to spin - without losing the tpi.