I do not know the size of your boat or what room you have but a tool I have been using since the mid-80's, now on two different boats, is an inexpensive Plotting Board with a Pantograph fitted with an adjustable compass rose (see Weems & Plath Chartkit Plotter
). With this, one can adjust the compass rose on the parallel rule for the amount of variation shown for each chart one might use and easily plot one's courses/headings in magnetic but easily translate them to True headings. The board can be laid out below, and, when necessary or desirable, taken into the cockpit (assuming one is using water proof/resistant charts). This equipment is very useful, particularly, for example, with Explorer Charts of the Bahamas, which report some significant differences in variation from locale to locale based upon local magnetic anomalies (which may account for some of the Bermuda Triangle mysteries). With this, you only need to work out your deviations and keep a little table, perhaps laminated in plastic, on a lanyard at your helm as we do.