Having a text formula update automatically across multiple sheets when I add additional information

johnsboxftm here is a file with the example. You will have to create a separated table to store the cities/tax rates (Sheet1, in the attached file), and then you just refer to it with your main table, by using the VLOOKUP formula.

For the VLOOKUP, you will use the following parameters:

City - The city that you want to find in the cities/taxes table (in the attached file, Sheet2, column B)

Range - The range of the cities/taxes table (in the attached file, Sheet1, A1:B36);

ColNumber - The number of the column, in Range, that you want to return the information, it means the tax rate (in attached file, Sheet1, column b)

RangeLookup - For your use case, set this parameter as false.

So you will build the formula in your file like this: VLOOKUP(City, Range, ColNumber, FALSE). 

Let me know if you need any additional help with it.