Evidence-based design always being best, between showers I managed to get the tripod and NEQ6 set up, with a bit of malformed roofing batten as a pointer that I could set at the level of the top of the 1500mm walls.
As a result I decided that I really needn’t worry about the wall height and my original design wouldn’t need changing. Obviously in the second photo there’s sky that will be obscured by the wall, but actually that’s below 30 degrees and only going to be visible for a short period of time so not really worth worrying abou?t.