Grrr... further proof of the need to proof. Foolestroupe was attempting to point out one of my errors: to move up (in pitch) on my chart you move right, not left; my first (and most crucial) instruction was wrong, though I hope the rest was correct. And my abject apologies to Foolestroupe.

To to recap:

If you capo and play as if "in key X", to find the real key (= harp key):
move right/"up"/thisaway => (one half-step for each fret)

To find where to capo (real key to "as if in" key of your choice):
move left/"down"/thataway <=

To find relative minor for a major key:
move left/down (<=) three half-steps.

To find relative major for a minor key:
move right/up (=>) three half-steps.

To play key X in cross-position:
move => 5 or <= 7, and that's the harp you need.
(Standard harps only, NOT Melody Makers)

To find the cross-harp key of a standard harp in key X:
move <= 5 or => 7.

(Now, someone check this over and make sure I haven't slipped up again.)

You can play minor tunes on a major harp, as long as they don't involve accidentals (particularly, major 7ths). To find the harp you'd need follow the rule for finding a relative major: move => 3 half-steps. Then start the scale two diatonic notes lower (on a standard Richter, this will be a ghastly doubly-bent note!)

With Melody Makers (only), you have another option: move <= 2 half-steps to find the harp, start one diatonic note higher, and bend down draw 5. Why would you? Because you sometimes run into minor tunes which also have major 6ths. And you may not have the "up 3" harp to play that key.

Of course, there are also harps in special minor tunings.