Most monitors are specifically designed to NOT lay on their sides. *ESPECIALLY* active monitors. Do NOT lay them on their sides unless it's specifically suggested in the manual. Most (even active units) *can* be inverted completely (pretty normal in surround rigs).
Anyway - You generally want the lower edge of the tweeter around ear level.
You want your head to be .38 of the long wall from the short wall. You want the speakers to be far enough away *from* the short wall as to not build up too much low end. Figure you want at least a foot. At least.
SO - a 15' x 11' (just as an example) room: .38 of 15 is 5.7' That's where your head goes. You want the speakers at least a foot from the wall, plus the cabinets. That'll put them around 3.5 feet from the mix position. 3.5' apart and you have your equilateral triangle.
Have them intersect maybe a yard or more (or a lot more, as I'll explain) past your head. I generally shoot for around the 1/4 mark of the rear wall. In the example case, you want to draw a line from the right tweeter to the right 1/4 mark on the 11' rear wall - Around 2.75' from the right wall.
Of course, the room is going to play a big role in all of this. In the "example" room, the rear wall would have to be VERY absorbent across the spectrum from the very low end (bass traps in the corners) all the way up to the upper mids.
