Parallell stripes? Don't cut the angles for everything, there's no possible way to measure everything, at least unless you're charging him for about a hundred sets of stripes so you can adjust and readjust until they're right. Just give him a roll of the stripes, pre spaced with transfer tape. He's got to be practical and understand there is a little hand work to installing them and he'll have to trip them to shape.
The good news is they are straight, they don't curve or anything, and a Mini is about the easiest car to install stripes on if you know what you're doing.