I know that, however if you shoot for a minimum of 528 feet, you should be okay. The issue was the "I can't find any rules to the distances of placing caches" when they've placed several. It's not hidden.
I get emails all the time asking "Is this location okay?" or "I know there's a cache nearby that's probably too close, but can I place one here? [gives coordinates]" Had one of those yesterday, as a matter of fact. I don't have a hard and fast rule of distance, every situation is different. If they provide coordinates, I'll give a yes or no. If they say "Within a couple hundred feet" I let them know I need more info, mostly exact coords are good.