Modern hackle capes and saddles are marvels of genetic engineering. But if not correctly sized prior to tie-in, they simply won’t do what they’re supposed to do.
The easiest way to size a hackle is with a quality hackle gauge, that’s usually attached to the stem of your tying vise. Here on the gauge the hackle fibers measure out to a size 14, in other words, their tips extend to about halfway between 12 and 16. The light and dark zones on the gauge help to accurately see the tips of different colored hackles.
You can also size directly on the hook - as with this size 14. Notice the fibers are roughly 1 1/2 times the hook gap in length, just right for a standard dry fly.
In most cases, measuring is easier if the cape or saddle is removed from its card. Measuring the feather before it’s plucked from the skin makes a lot of sense. Once again, you can measure it on the hook as well.
I actually like to use the hackle gauge off the vise stem for measurement, only when you’re sure of its size do you pluck the feather free.