Hi all,
I have a question regarding compass deviation. I've just started working through Bob's Nav Workbook and have realised that I don't know how to calculate the precise compass heading.

Obviously, when the compass deviation card says something like this:

For M: 030 060
Steer C: 028 058

I understand that the course will always be 2 degrees less. E.g. for 025 M, you need to steer 023.

However, what if the numbers are not separated by 30.
For example, in the Nav workbook:

For M: N 030
Steer C: 000 028

For 022M it says steer 021 (I wrote steer 20)
But for 027M steer 027. (I wrote steer 25)

I made many mistakes due to not knowing how to round up or down here.

Please help if pos.
Cheers,
Greg

Here's another example

For M 030 060
Steer C 028 060

The compass heading is 2 under for 030 and is correct for 060. So halfway between it would be 1 under. Nobody is interested in fractions of degrees on a compass, so if the magnetic heading is closer to 030 than it is to halfway, use 2 under, if it is closer to half way than to either 030 or 060, use 1 under and if it is closer to 060 than halfway, make no allowance.