[aerohead]

Your original fitting of the template is the correct one.The overall height of the RAV4 must be the same as that of the template for the roofline to be trouble-free.
The underside of the RAV4 will determine what kind of belly and diffuser you can use,and once this is determined,you can do what you can to project the new underside rearwards at either 4-degrees,or 2.8-degrees,depending.
The body sides of the RAV4 will also determine how you can handle the plan-taper of the elongation.
For the rear mods to function,you must provide the highest quality of flow from upstream.(flush rocker panels,rear skirts,rocker panel extensions rear of the rear wheels,complete belly pan,with diffuser).
Any elongation will cut drag.You decide what your target is.Also,if you do the simple,vertical Kamm chop,the tail can be modular,adding more length later if your hunger for additional drag reduction increases.