My outlines are not set in stone. Nor should yours be. Often I’ve outlined a short story to realize as I was writing it the outline would be more suitable for a tale of over 3000 words, way too long for most competitions.
In a sense I’ve not been sorry about this as it makes me re-evaluate my outline and story and make it sharper, tighter, bring it down to the bare bones as most short stories are around the 1500 to 2000 word mark so they have to be to the point. (I love being able to set a word target in Scrivener and watch the bar change colour as I reach my word count. I also find this simple device keeps me writing. There is something that makes me want to see that bar drawing reach its goal and turn "green" so I keep writing until I get there! I've also recently discovered if editing a longer piece you can still set the word target to the required level and watch your word count come down!).
But the biggest advantage of an outline is you can work out ideas first rather than start writing a tale and find you run out of steam. I also don’t allow myself too long to do the outline. A couple of sessions to work it out and then I get writing… Outlines are there to help you write the tale, not to help you procrastinate (and there are so many fascinating writing blogs and books that can keep you away from what you should be doing - writing!).