Airplane's compass mag dev

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