Are you asking about setting the decimal places in your formula?
The simplest way is through the format editor. Right click your formula --> format field --> click Number tab -->Choose the number of decimals from the list.
You can also click the Customize button and set "Decimals" and "Rounding" values to the same number.
If the number of decimals comes from one table field, you can use the conditional format buttons beside the "Decimals" and "Rounding" values and type this field in the formula editor.