Just found this thread and had asked the same question myself, Orientation of UV Sensor
Comparing the UV readings I was obtaining, compared to those by the BOM, it was 3-4 lower than it should have been. Tilting gives me readings that are directly comparable. Must be an issue with how the lens on top of the sensor collects the radiation. I plan to mount the UV sensor at around 52.5 degrees, the sun angle at the equinox, half way between minimum and maximum angles. Will see how that goes in the middle of summer in terms of accuracy / comparison of the UV reading.