See, this is why I think the HV mode of vector movement would work better. With that you'd just plot how you thrust and then work out how it affects your actual movement.
Lets say you've got a movement of 4 North and 2 North-West, and a ship with four MPs. You decide that you want to slow down (perhaps a massive minefield, or you'll be passing through the arcs of a dreadnought if you continue at the same speed and heading). You write down, in the ship record sheet which you use for screens, how you want to distribute your thrust. In this instance, you decide that a plot of 3 South and 1 South-East would be a good idea.
The three South points negate three North points, and the sole South-East point negates a North-West point. You end up with movement of 1 North, 1 North-West.
Simple, and allows for 'reverse'