If a rifle shoots about 2.5 inches to the left at 100 meters, how much should the front sight be moved? And it should be moved to the left, correct?

I'm guessing not more than 1/2mm, but am not sure...

Any advice would be appreciated!

From Brownells:

"Formula

Amount of Error X Sight Radius = Sight Correction Needed
Distance to Target

If you are shooting low, you would replace your front sight with a lower sight, and conversely, if you were shooting high, then you would replace your front sight with a higher sight. Add or subtract the number of thousandths needed to the actual height of your sight and you will have the correct height of the replacement sight.

Example
At 100 yards, your shot is 6" low and the distance between your front and rear sight is 19.5".

6 X 19.5 = .0325
3600

Since you are shooting low, you would need to lower your front sight by .032"."

This should work horizontally too.

Thanks for the tip New_Guy.

This will take u to a lyman page. There is a chart there for adjusting sight for up and down. Will work for left and right also. Yes, move your front sight to the Left. YOu will need the pdf reader to see pageLYMAN