I found that for the subtractions the quickest way is to add the 2 last numbers as above. - in fact just check the last digits - no point doing the whole sum if the last digits don't add up - it can't possilbly be correct.
For the radar I just kept track of the N/S and E/W components, and muttered them to myself i.e
20N ..... 20N
10W......20N, 10W
30S.......10S,10W
30E........10S,20E
And then reverse them at the end. i.e in the example above put the cross on 10N, 20W and press the button.
For the NE, SW etc vectors you can do the same but you need a bit of trig - i.e 15NW is the same as 10N, 10W. 20SE is the same as 14S, 14E
I'll let you work out the numbers - but I just memorised them all - they are all pretty similar - and a bit of rounding up or down doesn't hurt.
You may find a different method that works - but that worked for me.