Note: I will not be updating normal numbers, you will have to do it if you want to stay up-to-date.
Heroic version: https://wago.io/o4a_CAPuz
Mythic version: https://wago.io/YESpjjVz4
Numbers are specifically for Shadow Priest. Note: if you would like it for another class, change the numbers in Custom Options to your class' numbers (parse percentages).
The values will need to be updated manually, as often as you would like, to ensure some level of accuracy. You can find the numbers for each boss in the Custom Options tab. i.e. Entry 1 = The Tarragrue's percentages.
Example for Shadow Priests:
You can find the values here: https://www.warcraftlogs.com/zone/statistics/28&class=Priest&spec=Shadow&dataset=0&difficulty=3#difficulty=3&boss=2423&class=Priest&spec=Shadow&dataset=100
You select the boss on WarcraftLogs, then in the weakaura settings (under the Custom Options tab) you would find the boss' Entry in the Entry dropdown.
After you do that, you are ready to input the max and the percentages values (to save, make sure to hit Okay or hit enter when you update the number values).
You repeat this for each boss and corresponding Entry.