I use a 3d6 RNG, so personally anything that needs a 5 or more is a given, and a 15 or more is the sort of 'only option left' long-shot.
DnD, I actually don't worry about it. Flat-curve D20 is so damn swingy that I don't consider
anything a foregone conclusion any more
![Big Grin :D](./images/smilies/biggrinyellow.gif)
.