I also recommend making several correction cards.
Like:
Everyhing on - normal operation
Just one radio on - you know, generator / alternator failure trying to get somewhere on battery alone.
Nothing on - you know - the battery ran down in the case above and you are trying to dead reacon home.
Also don't forget, if you are flying an airplane with a manually adjusted Directional Gyro.
CH +/- Deviation = MH
So when you are setting the DG apply the compass correction card before setting the DG.
Bil