First, open up your MSBT options by typing /msbt and select Triggers. Click the "Add New Trigger" button - for my output message I put "Low Focus! (%a)" (basically copying the output for the low mana and health triggers).
Then open up the configuration for the new trigger by clicking the gear icon.
|I specified hunter for the trigger class. I'm sure it's unnecessary but I like to be thorough.|
The event we're looking for is "Power Change". Once you've selected that, you need to set your conditions. As you can see above, the conditions I selected were Power Type Is Equal To Focus, Unit ID Is Equal To You, and Threshold Is Less Than 20. (Maybe I should change it to 30 though... I'm still playing with it.)
I also added an exception:
That was basically so it wouldn't keep dinging at me as long as I'm at low focus. I only need to know once I hit the mark and then I can switch to steady shot or pop fervor or whatever.
I wanted an audible alert for it as well as the visual notification so I set that too. To access the font/sound options, you click the crossed tools rather than the gear next to it in the trigger list.
I used the low mana alert because I'm used to it. You can always create your own sound files and use those instead of the presets though.
And this is what the visual cue looks like:
|The yellow text above my head tells me my focus is below my trigger condition mark, and what percentage it's at.|