I hope I have my math right: A 6" f/8 scope should have a focal length of about 1200mm. So a 5.7mm eyepice will give you a magnification of about 210x. This is not a huge magnification, so that would probably be a good eyepiece for your scope. <br /><br />When I say good, 210x is a good magnification for planets, planetary nebulae, and faint galaxies -- that is, small objects. A better, all-round magnifcation might be more like 80x, which would require more like a 15mm eyepice.<br /><br />Of course, your ability to see anything crisply at 210x will depend on the steadiness of the skies, whether your scope is properly cooled down to the ambient temperature (2-3 hours for Dobs, generally), and whether your optics are collimated.