There is a shortcut key, Ctrl+N
So, when your hand is on the keyboard to change C<whatever> to first unused X, just type
X0<enter>Ctrl+N
No need to move your hand back to the mouse.
That probably reduces the time from 0.37 seconds to 0.25 seconds, an increase of 33% in productivity!

The other thing you can do, say you have a bank of C's where your alarm bits start, say C1000. Just type C1000<enter>Ctrl+N and you get the next alarm bit.