The "old school" of thought is 2 x lpi of the printer, real halftone printing on an offset press will use a lpi of about 80 to about 175, so you need about 160 dpi for newpaper quality printing, 350 dpi for the best quality printing (like in the brochures you get in a new car showroom). Lasers and inkjets don't really have an lpi, they have a "assumed" lpi, ranging ing from about 52 for 300 dpi lasers to about 120 for 720 dpi inkjets, so a dpi of about 105 is correct for a 300 dpi laser, 210 for a 600 dpi laser or inkjet, 240 for a 720 dpi inkjet. You should never go higher than that, the above will do the best output the printer can do, the "reformed" school of dpi uses a dpi of about 75% of the above numbers.