I thought ab out that but when you get to measuring its usually less. You have to look at the whole picture: the frequency response of your speakers, where you are crossing them, the slope of your crossover(for the overlap and where they fit), and the comb filter distortion for how far apart they have to be.
Like i said, for the system I developed for me I could only use 3/4 inch domes that had the domes literally silk to silk. I would have liked to put my 3.5 inch mids closer together but i also wanted each one to be separated physically completely from the others so that there was no backwave interference, and i wanted to make sure that the backwave for each was completely negligible so that nothing from the back of the speaker was coming back through the front to muddy the midrange sound.