Oh yes, that because you are using 2 points, and you cant derivate that, from 2 points..
So your solution is very very simple..

You must have have 3 points.. So.. if you have a head with 2 lights, you need on more in on the upper side.. like this:



()
||
()--[HEAD]--()

Thus you can draw a triangle.. and when you aproach , however they scale they on screen.. the distance between those, is the same...
But when you rotate.. the distance will cause the triangle to srhink.. horizontaly and the distance of the left to the right will be smaller but, the up will mantain..

You can calcultate that by the angles of the corners, or by dividing the side lenghts with the down lenth