How far apart do you position your speakers ?

Where you measure from depends on your speaker. On a 2 way it is normally on the top center of the woofer below the tweeter, on a 3 way it would be the center of the midrange. I have an MTM with a staggered tweeter.
Think about what it takes to time align your speakers and think of the center of that alignment. This will give you consistant results.

Timlub,

How about flat panel speakers?

Jim Smith arrives at the distance between speakers by measuring from the center of the tweeters. The distance to the listening position is from the tweeter to your ear. I'm sure there are situations where the 83% formula does not result in the best sound--every room and speaker are different--but I have found that this tends to work best when I've tried it not only on my own system but with friends as well. Another interesting "tip" in Jim's book is that you should sit fairly close to the rear wall--say between one and two feet. I did not sit anywhere near the rear wall in my old listening room but since moving and setting up in a different space I am now 16" from the wall behind me and am getting very good sound.

If you sit that close to the rear wall, I suggest you have some absorptive room treatment directly behind the listening position.

Sitting close to the rear wall can help bass response at times. There is no getting around that the reflection from the rear wall arrives very close to the direct sound of the speakers. Absorption will reduce the amplitude of the reflection but arrival time is still the same. In my case, this decreased resolution significantly by compromising impulse response and comb filtering. Diffusors may spread out the reflection arrival time but sitting very close to diffusors can be problematic too.