hey sam and jood,
the formula that jood mention here is for the site to site distance to avoid any interference as u can see d proportional to reuse pattern , if the reuse is less the distance has to be reduced ,.
But u must have capacity of the site under consideration to i,e u might have a D very large but the sites would get congested ,
For me it has to be a compromise of capacity + coverage range u should max erl per sq Km of the site to get the best quality of network