I don't think you can use ratios to kit together a great erotic story. Stories are art. The story should move the plot forward. Some of the best erotic stories I have ever read have had teasing and half measures for the first 80% before the sex at the end. Some have little bits of sex sprinkled throughout. If you are looking for a rules you can tack up on the wall to improve your story every time, and I mean EVERY time, I would say there are a few: 1) Worry about grammar when you re-write your story 2) Re-write your story - print it out and make corrections/notes with ink. You'll notice so many neat improvements doing that. 3) When you re-write your story, see where you can shorten things and do it. I read Stephen King's book "On Writing". So many great tips. One thing he said that stuck with me is something like "kill your little dearies" meaning just because you wrote it, don't be afraid to cut it down to a size that still makes sense. You'll really make it less boring.