And to make things even more complicated, felted items shrink more at the edges than they do in the middle, so your swatch only gives a very crude idea of how much shrinkage to expect. It is more helpful to assess the qualities of the final fabric.
If you really, really had to have a good idea of how much shrinkage you might expect with a given yarn/combination, you could knit a really big swatch (maybe 8" square) and then with non-felting cotton thread mark off a square 6" x 6" in the center, and then felt. The final measurements of the thread square will give you the best idea of expected shrinkage.
Can I just say, I've never done this, nor can I ever imagine doing it, but at least I could if I really wanted to.
I just use the old rule of thumb that in stockinette or garter, you lose about a third in height and a quarter in width. And in multi-directional knitting (like entrelac or feather/fan) you lose about a quarter in both dimensions.