Emmett v Sisson only helps where what is at issue is not the extent of the land over which the right of way was actually granted, but whether the grantee is entitled to insist on using the full extent and where the points of access are. In your case it is more a question of establishing the extent of the grant. That is a question of construction to be answered by referring to the instrument creating the right and, if necessary, to the physical layout.
Assuming that as to description the terms of the grant say no more than it is exercisable "over and along the strip of land to the south of the land conveyed" and that there is nothing else in the instrument which may help to identify the strip of land (apart from the plan), there is no guidance from the words as to where the strip of land is other than that it is to the south. "Strip of land" is not terribly helpful as a description compared with, say, "drive". However, "strip" implies something which is significantly longer than it is wider. The purpose of the right of way has to be to get from the property to the public highway and there have to be at least two termini, one on your property and one on the public highway.
Looking at the plan, we can see where the terminus is with the public highway. GIven the layout and that the right is granted over a "strip of land" another terminus has to be immediately to the south of the garage gates. The dotted lines on the plan appear to show a reasonable route between these two termini, without us being able to conclude that the dotted lines actually indicate the route, or indeed anything at all. What I do think we can conclude is that the intended route cannot deviate too far from the apparent route shown by the dotted lines. If the dotted lines correspond with the boundaries of an area which is clearly demarcated then there can be little doubt that that area is the strip of land. Even if the plan is inaccurate, given that the text of the instrument does not refer to the plan, the strip is going to be the demarcated land.
If the land is not demarcated, the strip has to be somewhere and cannot simply float to suit the convenience of either landlowner. If the same route has been used for 30 years then that route has been accepted as the land over which the right is exercisable.