At 1 to 1 on the screen, 150 is about right. Screen displays are about 150 DPI. A lower screen DPI is 75, but most are in the 100 to 150 range.

For printed output, you should see a slight improvement at 300 and up since printers can do that. That's also an issue that scanning at 300 and printing at 300 may make it worse than scanning at 1200 and printing at 300. The computer's software can take a 1200 DPI scan image and dither or decide if there should be a dot on a character edge. In the case of HP RET capable printers, the printer driver could vary the dot size along the edge, but at 300 DPI wouldn't have the information to make such a choice.

You'll find the real answer is trial and error.

Bob