I generally mix pumice with organic soil. I do, however, layer up. Which I have been advised not to. I use 100% pumice for the bottom 1/3 up the pot. Then when I reach the middle I mix the pumice with organic soil composed mostly of medium fir bark (that I crush myself so it isn't too big). Then at the top, I use purely organic soil; crushed smaller particles of medium fir bark. To my understanding, the bottom layer aids drainage. The middle has good draining, yet retains good moisture for the roots. The top? retains moisture, and keeps moss healthy looking. Again, I've heard going 100% pumice is better, and layering is not really that great, my trees are doing great on my soil though. I post this, because I would like to hear criticism about doing this. I've read this way of layering soil in a book, but that doesn't mean they aren't wrong.
My junipers have more pumice than as explained above. Boxwood is potted on the explained above. Forsythia has more organic matter (apparently they like more moist soil?)