You can build Custom Actions like
Increase Selected Track(s) Volume and
Decrease Selected Track(s) Volume. Go to main menu
Actions->Show actions list... and left-click on
New... button, a new window will prompt to build a Custom Action. Set a
Custom Action Name (increase.... or decrease...) and on Filter type
nudge volume for selected tracks, you'll see now two actions only. Depending on what Custom Action you want to build first, doubleclick one of them as many times as you like that action to be performed (how faster you'd like to increase or decrease), example:
Each action inserted increase or decrease 0.05dB's above example decrease track(s) volume on 0.20dB's
After building each Custom Action tick
Consolidate undo points and click OK. Go to
Shortcuts for selected actions and click on
Add... to assign a key to it.