It is pretty much that, if you live close enough to spawn. Imagine a ring where 4 of the inside points are 500, 0; 0, 500; -500, 0; 0, -500 and the outside points are 1100, 0; 0, 1100; -1100, 0; 0, -1100. If you drew a circle from those points, at least some part of all 3 of the strongholds (they can grow out of the area afaik) should be in it, every time. The only exception should be custom built servers or possible mod interference (mystcraft ages f.ex.)
Honestly if you were being thorough you might want to head out 800~ blocks, then throw the eye. 500 works just as good though. It's "easy" but if you are unlucky it could still theoretically take you a few hundred blocks of searching, especially as the eyes are not very accurate sometimes and like to go all over the place.
What this does do is stop you from wasting possibly dozen's of ender eyes if yo uare either inside the ring or outside. If someone comes with new evidence proving this all wrong then disregard it, but last I understood this was how strongholds spawned.
It makes sense in all honesty. A programmer would have to add a limit on where it can spawn to make sure it doesn't spawn 10000's of blocks out, assuming they weren't extremely sadistic (ok I would if I were programming it /admit), but I am not sure as to the validity of the triangle theory first hand as I've never bothered to work it out. A creative server and some maths could prove it in <30 mins work though.