It looks I got another calibration for UV this morning. In the end the max UV matches the national one almost exactly. (sky max 7.1, national max 7.6) . However the values in the late afternoon are almost double of what is expected (sky 3.5, national 1.8).
I already had a feeling that the values were too low. That seems to be corrected for measurements around 7 but for the lower values (like 3.5) it is way off.
I’m wondering of the scaling is only a simple multiplication, which will never produce a good result, judging from these measurements.
edit: currently sky 2.2 vs 0.8 national. still clear skies.
(note, clear skies, no influence from clouds)