In theory, interpolate. In your example, steer 078.5 degrees.
In practice, you can't resolve that on the compass, the local magnetic
variance isn't constant, and the winds don't always do what the
forecasters say, so aim for somewhere between 075 and 080 until you see
the next landmark on your route plan.

Curt

On 3/24/2012 7:40 AM, George Las wrote:
Morning

I have to preface this post by telling you that I am not a pilot, nor
will ever be due to a physical impairment, but since my senior year in
high school (1971) have had a very keen interest in aeronautics.

Now for my question regarding magnetic heading and the aircraft's
magnetic deviation.

If I'm not mistaken when "swinging the compass" is performed it is done
in 30 degree increments. Right?

Let's us say that after swing the compass it is found that the
correction for 060 degrees you would steer 062, and for 090 you would
steer 095 degrees.

So in order to find the calculated compass heading what deviation would
be used if for example your magnetic heading is 075 degrees?

What I'm basically asking is how to figure the deviation between the 30
degree increments?

Thanks in advance.

George