It depends on your speaker's overal and tweeter height and the mounting height of your screen. If you are elevating your screen a little high, usually due to making room for other rows, then I'd mount the speakers as low as possible within the screen area. If it helps, you can mount the speaker upside-down if that helps get the tweeter lower or if it's a tall channel. If you're not mounting your screen that high then b) and a) are closer together and less of an issue. One of the many advantages of an AT screen is that you can mount the speakers wherever they perform best, independent of the screen = acoustic best practice. But if you need a guideline, I'd say if eyeline is about 40" from the floor, the tweeters should be from maybe 36" to 56" up.