It sounds like you need to use a plumb-bob or level to find “straight up” at the port-side bulkhead. Then you need to make a shim that matches the angle that forms with the bulkhead, and use it when mounting the compass on the bulkhead. There is no set “angle” that anyone can give you, because each boat will float a little differently. Some may have more chain in the anchor locker, or stow diving gear aft. This is a custom installation.