your mic should have came with a gauge aka "standard" to set it.....or you are using the wrong mic....
what make is your mic? mics dont just go wrong...either you are turning it too hard or you droped it....
to use a mic proper just spin the dial very very very lightly and wiggle the mic around the part untill the slightest pressure doesnt turn the spindle anymore...that is your accurate reading...when i use mics i use 1 finger very gently to spin it.
im a machinist and our shop standard is .0003"....very very tight tolerance shop as we are iso9001 certified....
i think your doing it wrong.