Magnetic fields and EMF are being confused. They are very different in how they affect a compass
. Lead will stop EMF but not a steady magnetic field such as the Earths magnetic field. DC currents create steady magnetic fields, which will destroy a compasses accuracy and lead will not stop it. Besides, if lead really did stop a steady magnetic field then why would you want to put it anywhere near a compass
? Only ferrous metals affect steady magnetic fields.
A deviation card is considered no good if it generally has any corrections above 15 degrees or so. This is what I have been told by compass adjusters as far as the Coasties are concerned. I have seen compasses swing over 30 degrees when exposed to DC caused magnetic fields making any sort of deviation card worthless. Really the best thing is to get a standard compass and have a professional make the adjustment and create a deviation table for you. You can then put the steering
compass where ever you want and you won't have to worry about inaccuracies. You can then make a second deviation table for the differences between the steering
and standard compasses.
You can find a better and more detailed explanation in Bowditch.