Help! I have been using Ashenhurst's Rule with Peggy Osterkamp's Use Rule to get a warp sett for particular yarns and interlacements, and they've been great tools. But I don't know what to do for my next project: a supplementary warp (using a double two-tie threading) and doing pick-up for the patterning, a situation where I will be consistently using two yarns of different weights in the warp. Do I figure out the setts for each yarn separately then average the two answers to get a final sett? Or do I base it on the heavier yarn only?
I know that Ashenhurst's Rule is useful (a prescribed method for determining sett based on the yarn's thickness and the number of actual interlacements per inch) but I've never actually used it. Instead, I determine the sett for plain weave for the particular yarn (either by doing a wrap or using the Master Yarn Chart) and then guess at how to adjust for twills (depending on float length) or other weave structures (doubling for doubleweave, for example) or other factors (desired fabric density). Then, if I discover my guess isn't right based on my observations when I start weaving, I cut off and resley (any change is not likely to be very great).
So, if you are using a supplementary warp with a double two-tie threading, your ground cloth is going to be plain weave and each supplementary warp end will alternate with a ground warp end. That means the ground warp needs to be sett for plain weave (24 ends per inch for 10/2 cotton, say) and the supplementary warp, since it alternates with the ground warp, will also be 24 ends per inch for 48 ends per inch overall.
However, the density that can be achieved with supplementary weft by beating is easier to achieve than the same density using a supplementary warp (the thicker the supplementary warp, the more difficult). So I might choose to make the sett slightly more open (20 ends per inch for both, for example). This way, I'll be more able to beat the fabric to square the design if that is desired (achieving 20 picks per inch with the single weft instead of 24).