answer to your question: Suppose you want to steer a TRUE course of 180 degrees (same as point telescope to due south.when lining up)
The Magnetic variation (declination) in your area is (say) 12 degrees East
Using the rule "Variation East, steer least" you would set the COMPASS course to read 180 - 12 = 168 ie when the compass is reading 168 degrees, it is really pointing due south 180 deg.
If your area like around Perth is say 4 Degrees West, "...Best" then to point due south (TRUE) you would set COMPASS to read 180 + 4 = 184 degrees. Unless you are on a big ship or bang next door to your shed which is full of scrap iron, and old car bodies forget the adjustment for "Deviation."
As you can see from the chart, magnetic variation makes a BIG difference, and can easily explain why some people may be way out with polar aligning their scopes.
Lucky if you don't live on Magnetic Island - it completely confused poor old Captain Cook - sent his compasses haywire!!!
Hope this helps
