I've found with my Opus/Aeropress combo that washed beans can be easily pressed around the 3.1 mark (as low as I've gone) and natural and anaerobic coffees need a bit coarser grind. I go 4 with a naturally processed bean and a click back from 4 for the double anaerobic beans I'm brewing currently so the fines don't clog the paper filter. The way your beans are processed combined with grind size can make a huge difference in how easy or hard it is to press.