You do not need to calibrate the compass unless it prompts you . A compass interference message is not a prompt to calibrate but a notice that you need to move away from whatever is causing the interference .
Check the red arrow on the map that indicates the Mavics position . If it is pointing in the same direction as the Mavic itself then the compass is fine .`
After arriving in Scotland last week from the US, I pulled out the Mavic and it was good to go! Flew it 12,000ft over water to an Island and then back with no issue. I'm getting 20+ satellites on every flight. Love this thing.