Store and read the urgency threshold in the taskwarrior config settings.

Look at https://github.com/HiveMinds-EU/tw-install/issues/83 before fixing this.
As explained in: https://groups.google.com/forum/#!searchin/taskwarrior-user/brutus%7Csort:date/taskwarrior-user/dsOzdYSKaM4/h0SwTFvjBwAJ

What about a config setting?

Have a look at https://taskwarrior.org/docs/hooks2.html, and you'll see a hook is passed the location of the .taskrc file as an argument. So you could search that file for your setting.

This also works from CLI:

[:~] $ task _show | grep custom.value

custom.value=10.3

However, I'm not sure you can call the task binary directly from inside a hook, you'll have to try that and see if anything freaks out.