So over a distance of about 20 inches you are worried about .001. that's already pretty dang precise. have you taken a micrometer and measured something .001 thick? a piece of paper is 4 times thicker. how long of a part do you normally work on, if it's 10 inches the error from side to side would be even less. too many people see numbers on a dial and if it isn't zero for everything they think it's no good. you need to learn about acceptable tolerances. you are better leaving well enough alone, you are likely to make it much worse scraping away yourself but it's your mill and you gotta have zeroes right.