[tbrownaw]

That is, you can distinguish lines that are about 1/150 inch apart.

But for a display, you want to be able to have diagonal (or curved) lines at 1/150 inch apart, with pixellation artifacts that are small relative to those lines.

That probably translates to something like 450ppi (line width of 3px, so the artifacts should probably be about 1/3 of the line size).