Let Photoshop Do Your Resolution Math
You don't need a calculator to determine how much resolution you need for printing to a particular line screenâ”Photoshop will do all the math for you, right inside the Image Size dialog. Here's how: Open the image you want to print. Go under the Image menu and choose Image Size. When the dialog appears, click on the Auto button (it's right under the Cancel button). When the Auto Resolution dialog appears, all you have to do is type in the line screen of the device you're printing to and then choose a quality setting. Here's how Photoshop does its resolution math:
Draft: This just lowers your resolution to 72 ppi (ideal for onscreen use, the Web, etc.).
Good: This takes the line screen and multiplies it by 1.5.
Best: This doubles the line screen (multiplies it by 2).
When you click OK, Photoshop enters the math it just did into the Resolution field of the Image Size dialog.