We have a 2012 Plus 4 and have can not enter our garage for the break over point which is actually quite moderate. Our ground clearance problems are mentioned in several posts so I won't go into details but it has made me think about this issue.
I was thinking the best thing to do was to measure the minimum ground clearance (when two people are in the car) and make a wooden frame with two vertical pieces of wood to represent the wheels at the distance between the front and rear axles. The height of the vertical pieces is obviously the minimum clearance. Using this you can play with the aproach and departure slopes and eventualy use the frame to check everything is ok before laying the concrete.