A nice idea
Even if you are not travelling north you can just assume you do and do everything relative to your own course. If you want to get the target's real course you can just add your own one later (and calculate modulo 360).
Only for a whole attack run it becomes a bit unhandy because it would be hard to adjust for your own course changes. Also you're limited to only one map scale which might be difficult if you have to track targets for a longer period of time.
But these are minor issues that hardly touch the main intend of your tool, as I understand it. I will give it a try, looks good
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