optimal distance from wall for electrical box for pendant fixture?

I'm thinking about adding a pendant fixture above my bathroom vanity. I know I need some minimum clearance from the wall (half the depth of whatever pendant fixture I end up with) but I want to keep it as close to the wall as possible, as it won't help much if the light is behind the person at the mirror.
I don't have a pendant picked out yet, so I figured I'd post first and see if there is a standard distance from the wall I should observe when putting in the octagonal box- I have the ceiling open from another project, so now is the right time to put in the box, even if I don't have a fixture yet (and who knows, maybe someone one day will want to replace the fixture with a different style, etc.)
Any suggestions on distance from box center to wall?
Thanks! Keith
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Every light fixture has a different lighting pattern. Good companies supply such information. Ballpark WAG,,,,,,,,, 3-5 inches to the center of the box from FINISHED WALL include mirror or anything else when measuring.
I have not seen this done since the mid 1970's.

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