Without knowing the question you are having a problem with all I can do is give you a general explanation using a sample Compass Deviation Card.
The first thing that you need to realise is that when you navigate an aircraft by compass you are flying a compass heading, not a magnetic heading. Yes, the two headings are intimately related and use synonymously, but they can and often do differ by a few degrees. In effect, the required magnetic heading is corrected for compass deviation (error) on that heading to fly a compass heading.
Without getting into too much detail,
If you wanted the steer 360M, you would fly 360C - compass north and magnetic are the same
If you wanted to steer 090M, you would fly 085C - compass north is 5 degrees east of magnetic north (dev E)
If you wanted to steer 270M, you would fly 274C - compass north is 4 degrees west of magnetic north (dev W)
This is all straight forward if you want to fly one of the cardinal points, but what compass heading would you fly if you wanted to fly say 004M or 018M or 025M?
Looking at the calibration card and filling in the deviation gaps in increments of 1 degree, we get
| For | N | --010 | --020 | --030 |
| Steer | N | | | --027 |
| Dev | 0 | --1E | --2E | --3E |
004M is closest to N so you would use a deviation of 0
The required heading would be 004-000 = 004C
018M is closest to 020M so you would use a deviation of 2E (2 degrees under)
The required heading would be 018-002 =016C
025M is half way between 020M and 030M, so you would use a deviation of 2E or 3E (2 or 3 degrees under)
The required heading would be 025-002 = 023C or 025-003 - 022C - both answers are correct.
On paper you might say 025-002.5 = 022.5C
In practice you will have trouble hold a heading within +/- 5 degrees
