You can use Alt+Shift+VolumeUp/Down to change volume by quarters of an increment.
Karabiner allows you to change the default behavior of the volume keys to use quarter steps with the "Fine grained volume controls" option (it simply adds the alt+shift for you). Very useful with headphones that are too sensitive. https://pqrs.org/osx/karabiner/
The volume plugin has a more fine-grained control for adjusting the volume. The keyboard based function keys will increase/decrease the volume by 6/7. Now I can increase by smaller increments. Excellent.