The F descriptor is used in the form Fw.d in order to present REAL values without giving an exponent part. w is the width of the field and d is the number of digits to be presented after the decimal point. The field is padded out with leading spaces if required. Remember to provide space for a sign if the value might be negative.
If the field width is too small for the value to be output, then the field is filled with asterisks in order to flag the error. As an alternative, the G descriptor can be used instead of F. This works like F if there is enough space, otherwise it uses the E descriptor.