Is your problem an offset or scaling error? Is it a constant offset, or does it change with the analog value? Input 0v and 3.3v. Is the offset the same with both?If it is offset, add or subtract the offset from the final value.If it is scaling, change the 1024 value to adjust the scaling factor.
How accurate is your 3.3V reference voltage? How accurate is your multimeter?
Dear AMPS-N,When are you going to stop relying on the forum to do the most basic tasks, and start thinking for yourself?