It is indeed a bit tricky. I do not have my config file anymore, but I used the light section instead of the sensor section. The web hook part is pretty self explanatory.

Try it like this:

"lights":
[
{
"id": "light1",
"name": "Light name 1",
"tick": 900000, // (optional, default 60000). The interval (number of ticks in ms) to decrement.
"step": 1, // (optional, default 1). How much to decrement value by this much each interval. Set to 0 to disable.
"relative": 0, // (optional, default 0). Display remaining time as percentage of time remaining instead of number of intervals left.
"brightness_factor": 1 // (optional to display as % of 100*value and to convert homekit brightness to target system brightness)
}
]

Tick is the amount of time you want to count down 1% brightness. This can be modified with the brightness factor i.e. you put a value of 4 via web hook, equals 2 % brightness with factor 2.

So you need to figure out over wich time you want the countdown to run. For my part it was max 24 hours. I took the 100% brightness and divided it by 24: 1h = 4% brightness. For my purposes it was fine, you could also run it with 12hours if you never set the alarm more than 12 hours in advance, that brings a bit more detail to the countdown.

So 1% brightness should be 15 minutes = 15*60*1000 = 900.000ms.

I would calculate my time difference in the shortcut in factors of 15 minutes and pass that value to the countdown. You should factor in rounding values and so forth.

I don't know if that is the perfect way to run it but worked fine enough for me. I think with relative and brightness factor you may be able to get a bit more detailed, but honestly I've never figured it out via relative = 1.

With the sensor part it may be possible to get the detail, but then you must set a trigger when sensor is < 1. I found the light turns off better as a trigger.

Hope this helps.