Perhaps. but I have seen preheat type ballasts labeled 15W-18W-20W, so I
don't think it is that critical.


Those run all the lamps at around 15W, it's not critical, but you do
want to stay within a reasonable percentage of the nominal wattage
printed on the lamp. Many inexpensive "40W" fluorescent ballasts
actually run the lamp at closer to 25W which is why they're so dim
compared to a good ballast.