You don't have to be perfect. I mean the average mustanger will never be able to tell. The spacing on the cobra isn't even in reality. The top section has a bigger gap from the bottom stripe to the large bar in the middle of the taillight.
I just did mine the easy way. It's 3 stripes in each section. So i put one stripe dead center and then the other two dead center of the area below and above the center strip.
Yes, the middle stripe doesn't go all the way around the outside of the light. But i always thought that looked stupid. SO when i did my light up, i made the stripe wrap around.
Like i said, nobody really knows enough to be able to tell. Not like guys will come up to you with rulers and measure the gaps. I've gotten plenty of compliments on my taillights just putting the three strips in each section centered