Automatically Adjusting a Column with a Formula

I am looking for a way to make a formula column automatically adjust to the numbers in the cell, without having to adjust it myself.
Column B: W/H SUMMARY
Column C: TAX PC
Column D: Formula: =SUM(B3-C3)
*Please see attached document for a look at my excel file*

Before I entered the 1,000,000 in Column B, col. D showed 0.00. When I enter a number into col. B or col. C I want col. D to adjust to fit. For example, right now it sould show 1,000,000. BUT, when I enter 1,000,000 into col. C I want col. D to go back to the smaller size.
Thanks!