You know what, I don't even think of percentages, I think it's very old fashion, and robotic in writing, I also think it stilts the person as a writer to have a percentage of something; I've heard this before with action as the genre, a lot.
I'm going to personally say don't look for a percentage, and if you must, write first, tally percentage later. Some books need a lot of sex scenes, some don't; I've read stories where I was bored out of my mind waiting for the sex to come, I've read others where I was sick of seeing the sex, each with various percentages.
I think it depends on the story, and how you write it; if the story is developing relationships, sex shouldn't come in until that time where you're ready to get rid of the drama of "sexual tension". Something like two people who hate each other, end up banging each other in the end, or just two childhood friends that don't admit they like each other yet; those story may be limited in sex, and only happens during the climax of the story. However, if it's a rather smutty story, but still have a storyline outside of erotica, I say you should check your genre (let's say you're writing about drama, action, romance, and erotica); you should make a "line", do some drama points, then some action points, then the romance points, then BAM erotica, then restart and repeat. Of course, this can cause some stilted draw back too, but that way you should be able to tell when the reader might be ready for another love scene.
Until you get "perfect" at it, you'll know when it's time to throw in the sexy, and when it's time to write the plot.
Now, are you saying how much should you put erotica inside of a story of erotica: 100%! You can have sexiness right from the beginning of the book, to the end, and it's up to you rather you cool it down or heat it up. Sex scenes may very, but I think a good erotica book should have sexy things happening all the time, or at least a hefty percentage.