almost 100% of it is personal preference.
With the sight mounted further forwards, you get more of a "bead" type effect with less (visible) movement of the dot, and they seem to get on target faster.
With it further back you get more open space with the housing being less noticeable (not as big a deal with 2-eyes open shooting).
I like to have my red-dots about mid way along the rail... just the most comfortable way for me.
Check out what position you like it in most, and keep it there. There are no downsides to having it more forwards or further back.