Well I'm sorry, but "think of the composition | there is no simplistic rule, just tweak it until it looks good" sounds a bit mutually contradictory. I think I pretty fairly understand the thought processes involved in choosing where to place the horizon,, but having done so you can still change the frame in an infinite number of ways by raising the camera and pointing slightly down, or lowering and pointing slightly up. The horizon won't move in the frame but every element - especially the closest ones - will move relative to it.
Say I have a really interesting foreground rock. Do I put the camera on the ground and level, or do I put in on the tripod and angle it down? In the first case, the horizon will likely intersect the rock; in the latter, it'll be above it. The horizon and sky and distant elements will more or less be in identically placed in the frame. Which one achieves which effect better? Those are two different ways to emphasize the foreground; how do they differ?