Fitting a Reversing Camera

[notorious]

Because of curved line of the boot lid the rear lens is not exactly perpendicular to reversing direction. So camera doesn't look exactly rearwards if mounted flush with the lens. Deviation angle will be around 10 degrees. How is this compensated to achieve precise positionig of parking lines?

My camera was sold with a special ring that compensates that angle, but it doesn't have parking lines. It does have night vision LEDs though.