Unlike variation which is dictated by location and will be listed on a chart, deviation is purely environmental. To make things more complex, there is no single value for deviation, which is why deviation tables are used. The electronics and magnetism surrounding the compass will impact deviation. In some cases, simply turning the boat in a certain direction will align metals in the boat and impact the deviation of the compass. this is why on deviation tables the deviation will vary depending on the heading.
Any new addtions to the boat; VHF attennae, GPS/chartplotters, radios, rail mounted equipment, and even the rails and bimini frames can all impact deviation. Whether your electronics are on or off and how close the electronics are to the compass will all impact deviation. therefore deviation is almost unique to each individual boat. You can create a rough deviation table, by using a secondary compass, such as a good handheld to compare with the compass readings. Compare and record the deviation for each few degrees of change in the boat.
In all reality, unless you're travelling hundreds upon hundreds of miles, you probably won't see the effects of deviation. The difference of travelling 20-40 miles won't be that significant by a few degress when your inshore boating. worse case scenario, the mouth of the inlet is a 100-200 hundreds different than you intended. Unless you plan on running in zero visibility with no GPS or Radar, you'll most likely see the mouth of the inlet when you arrive. Wind and current will have more affect than compass deviation when navigating such short distances.
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