Part of the above problem is the definition of the words. There are 3 kinds of bearings: true, magnetic, and compass bearing. Maps use True North (pointing to the North Pole.) Your compass points to magnetic north and the difference between that and true (in your area 12 deg) is the "variation"
so the calculation you are making is to convert the "True"
heading (from your map) to your "Magnetic"
heading. Often that is enough for many boats. The third type of bearing is "Compass" and that takes into account the "deviation
" which is caused by the magnetic properties of the boat (this is only for fixed compasses and can not be calcualated to hand held or compasses that can move around.) You are probably ignoring this calculation. I do since it's less than 2 deg. and I can't really steer closer than that anyway.
Now back to your original question.... I have no idea. But I do know that using Navionics
charts on my Raymarine
can convert from True to Magnetic automatically. I have just ordered an iPad so I do hope it will do the same thing.