Brush tuning seems to be something that a lot of brush enthusiast are confused or intimidated by. Questions come up about it from time to time, so I'm going to do a quick guide to how I start the proses. Tuning a brush, is really just finding the loft that works best for you. That could be tall and loose with great flow threw for bowl latherers. It could be low dense and scrubby for a face latherer. But more often than not its someplace in the middle.
Whatever your preference, remember your brand new knot is not broken in yet, and this will only be a starting point for your particular brush. Sure it will start to break in as you do more and more test lathers, but most knots will take some additional time to fully break in. Again, this method will only find you a good starting place for your knot. You might still have to do some adjustment latter on when things get to where they are going to go.
Different types of hair, have different characteristics, so what works for one type or even knot might not work for the next brush you have. Differences in density, backbone, etc. will require different tuning to get the same face feel. For example in general three band hair will have to be set lower than two band to have equal backbone. As you add backbone, the tips might loose some of their softness, or the skritch might become much more pronounced. Furthermore different densities of the same hair will change flow threw etc. My point is that many brush characteristics will change with the loft, and you have to find your own happy medium.
Also, keep in mind that the lower you set the knot, the more backbone it will to have but the smaller it will feel on your face. Another concern for bulb shaped knots, is that they become more pointy if you continue to reduce the loft size. Most importantly remember that the feel and performance are more important than aesthetics. However, aesthetics do give strong clues as you experiment.
As example here is a 24mm TGN Ex Finest at its sock loft. Just set in the handle at the top of the knot bed. Pardon my old soap in the beehive ribs.
Here it is at its minimum loft (for this knot bed anyway). I should have left something next to the knot in the top down pic for perspective, but it is in fact smaller at the lower loft. Also notice how much closer the hairs are in the knot. Look how mean and pointy it is! Who would want to put that brush on their face? '
About a year ago there was a bit to do about one of the major manufacturers letting brushes approaching this kind pointy shape slip past quality control... oops.