8R is not 3:2 - 8 by 10 inch
Super 8R is 3:2 - 8 by 12 inch
Maybe that was the problem.
Oh to answer ur question.....
8 X 10 = 4:5 ratio
so.... if 1200px = 4 , then 1500px = 5 (am i right?)
so u need an image of 1200 by 1500 (not 1800)... as long as u have the 4:5ratio ya?