The scanner experts I talk to say that scanning line art at a resolution higher than 1,200 pixels per inch is a waste of pixels. The difference in output quality is insignificant. In fact, the only thing you will get more of is the number of megabytes in your file. If you are scanning line art that has some blurred detail, try sharpening during the scanning process. It will save you from having to fix it after scanning in an editing program.
Threshold determines how much detail is retained in light areas or lost in shadow areas.
Threshold value also allows you to convert gray tones to pure black or white. Pixels that are lighter than the threshold value will be converted to white, and darker pixels will be changed to black. To retain details in a grayscale original, the threshold should be set at around the middle of the tonal range of the image. Use a quality factor of 2 for screen rulings of 133 lines per inch and lower. Use 1.5 if the screen ruling is higher than 133 lpi.