I usually try to create a wider space between my capital and other cities because my capital will grow fast enough that it'll fill up a bit quicker and I want one epic city. but not so much between later cities. Most of the game, overlap is not an issue and when populations get so high that it is? Well, I just pick the cities I want to be epic and give them all the tiles. Others get stuck with less. Since this is usually the last 50 turns of the game you actually wasted less potential by overlapping then if you'd overspaced them as most of the game this results in unused tiles. Plus, having a city use up most of its useful tiles early is not a bad thing. Specialists are good for science and those cities often end up giving you extra Great Scientists and Engineers by the end-game. I often space by 4 or 5, but not very often as small as 3 unless I expect that city to be worse and just want to grab a few missed resources (such as fish or whales). I've found Greater than 50% overlap is significant and can result in worse cities as you start crowding fairly early.