when I tried to install a buddys, I measured my stock lights and after putting them on the buddys truck, the measurements went out the window! Just space them where you want them and make sure the bottom of the lights meets the curve on the top of the truck. Stand back and make adjustments.... Its easier than what you think and you will be happy with what you come up with.
I just found the center of the top of the truck and started from there, making sure all the lights were symetrical. Then once finished, measure the end lights to the end of the top of the truck and make sure they are equal. Stand back and check out your work.