That's interesting ... I never thought of a formula, I post either earlier in the morning (when I think people check their FB before work) and it's up all day or later in the afternoon. I've had pictures that show a reach of 50 and those that show a reach of over 800. I just figured it was because they 'liked' one picture more than another, rather than it being the time of day! Let me know if you figure out a formula though :-)