Mathematica has the cool ability to do symbolic tracking of numerical precision, for the ability to tell you when, for example, your differential equation solver is giving you meaningless results.
Because it depends on the algorithms you put a float through.
Addition has a maximum accuracy of 1 LSB. Makes sense: the last bit could have been "rounded off" and 1.5+1.5 == 2 (but really 3 should have been returned).
Subtraction has unlimited error bounds (!!!). Well, I guess there's 53-bits of a double-precision float. So subtraction can theoretically create 53-bits of error.
In practice, you need to keep track of the error bounds during the runtime of the program. Its not something that can be computed at compile time. After all, addition of a positive and negative number IS subtraction. (so some subtractions are additions: with accuracy of 1LSB. While some additions are subtractions: with unlimited error bounds)