If you will be adding up the mileage reimbursement amounts you might need to round the result of the calculation to 2 decimal places and that will require a custom JavaScript calculation. Using unrounded values can result with an answer that does not add up to the displayed value.
Using the simplified field notation also requires that field names meet a very specific format. The field names need to start with an alphabetic character and it cannot have any white space character in the name unless some additional coding is done. These restrictions are a result of processing the formula string with JavaScript and how the data string is parsed for processing.