First thing - do a 4-point compass swing using a known good external compass to compare with the aircraft. Is the 25-30 degree error constant on all 4 cardinals or does it reverse on some? There are North-South and East-West adjusters within the compass and if the error is constant on all headings, you shim the mount with brass washers to impose a fixed correction on all headings. Any good mIntenance company and particularly a good avionic tech should be easily able to sort this.