I've never plotted a mystery before. It's harder than it sounds, and much more difficult than a suspense. With a mystery you can't reveal too much too soon or too many clues. With a suspense it doesn't matter what you reveal to who and when.
I'm trying to catch up on my Agatha Christie and old B&W who-done-its. Some of them it's hard to guess who before the big reveal; some not so much, mostly because they drop too many clues or jump to conclusions rather than follow the leads.