http://www.luminous-landscape.com/forum ... ic=46664.0
So if you want to make 20x30 inch and the native resolution goes down to 180ppi when sized to print dimensions, then increase resolution by 50% by clicking on the print resolution button and uprez to 270ppi (50%) - in my view, if your going to 270dpi, then might as well take it to 360dpi.
Remember - this is Lightroom, your not producing a new image file at that resolution; its all virtual editing and output sharpening applied to a single original file including making a virtual copy of that image should you want to permanently retain those settings along with a print preset. Its great that Jeff is prepared to share his experience in this way and goes on to say
The types of images that benefit are images with lots of high frequency textural detail or strong high contrast diagonals or circles.
Thanks to LuminousLandscape for this information