It should be tied to the highest COL region (so that it does not economically limit geographic mobility) and it should allow someone living there to:

• Pay rent of a 1b/1ba apartment at current market rate with less than 1/3 income. As well as utilities (Internet included)
• Pay for a healthy diet at current costs of food.
• Pay for all medical care that could be necessary (this shouldn't actually cost anything but a small tax on the income).

And

• Pay for tuition and all costs associated with being a full-time university student while working 25% of the year. Or
• Leave sufficient income after expenses to save up for a market rate home in 5-7 years of working full-time.

(Note that this is all predicated on continued use of a capitalist economic system and would likely be much easier to achieve in ways not directly tied to wage in a more advanced economic system.)