If the post is all the way down, you should still have plenty of post left to file it down some. You may gain enough to get it zeroed. Posts are cheap enough if it doesnt work out.
Id use HWI's method and take it slow, with a couple of passes with the file and shoot and see.
“The enemy is anybody who's going to get you killed, no matter which side he is on.” - Joseph Heller
Beware of associating with "Cheap" Squirrels. They tend to be very narrow minded, hypocritical, and will gag you if you question or disagree. ;)