Alright so my idea was to just add a new tool to the "Add" unit menu called "Subtract".
...so the menu for the Add unit now includes this "Subtract" toggle. As you can see above, if the Subtract toggle isn't turned on, the add unit performs its normal end-to-end vector addition for unit vectors 1 & 2 (in this case vector 1 is Ownship and vector 2 is the Contact).
If you change the Subtract toggle to on you'll see the vector addition now shows vectors 1 & 2 starting at the Add unit origin and the resulting vector is shown to start from the endpoint of vector 1 and terminating at the endpoint of vector 2 (again, vector 1 is OwnShip and vector 2 is the contact). Notice the heading of the Add unit is now reported as the heading of the vector 307°.
Previously, if you had to convert true to relative (as in Prob 3 from
Chapter 11 - Maneuvering Board" of the US Navy Operations Specialist NRTC) you would've needed to manually do the vector calcs...
Here we see how you'd calculate DRM when the Add unit didn't have the "Subtract" feature.
Now with the new feature enabled we can do it like this:
So now we have the ability to convert from true to relative, in the example above you'll notice we're approaching the contact at a relative speed of 25kts and the contact DRM (direction of relative motion) is 307°.
Is this the sort of thing you were looking for?