In Photoshop, I was taught to start with the brush opacity set to 20%. Quite often when painting on a mask, different areas need subtly different amounts of density. This, when used with a Wacom, allows impressive control of the density, from just a little to quite a bit on the same mask. It's uncommon for me to need to change that opacity. I'm patient with the brush and if it takes me a bit of time to get to the density I want it's worth it. Should I go a bit far, one can always decrease the opacity of the layer itself. It's quite a bit more complex in Lightroom, with the advantage of all the other adjustments that can be made in Lightroom. I've waffled about quite a bit about how to use the brush in Lightroom, I've tried both ends of the suggestions in this thread and currently use low flow and high density for the same kind of control I have in Photoshop.