The %e format always outputs exactly one digit before the decimal place, and outputs a number of decimal places after that according to the format you give, such as 5 decimal places for the %24.5e format. So far that is 1 leading + 5 after decimal [in this case] + decimal place itself, subtotal 7 so far. It always always has a sign character for the exponent, so subtotal 8 now.
If the value is negative, then it will use one place for that. If your leading width such as the 24 in %24.5e is not wide enough to accomodate everything including the leading - then it will use an extra character .
Now we get to the question of how many digits of exponent there are. If I recall correctly, the rule is:
- if you are on MS Windows, or abs() of the exponent is 100 or greater, then 3 digits of exponent are used
- if you are on Mac or Linux and abs() of the exponent is 99 or less, then 2 digits of exponent are used
So take the subtotal of 8, add 3 potential digits for exponent, add 1 potential place for sign, for a total of 12 in this case (7 plus number of digits you specified after the period in the %.e format). No value will require more than that.