Radar covering south of the tip of Istria peninsula
According to the Windy radar maps in the bay of Kvarner never rains southern of the Istria peninsula tip. As well as in island Mali Losinj. Obviously this area is not covered by your radars.
Compare rain souther of the Istria with for Slovenian radar. Compare difference this morning between 9am and 10pm.
In addition to this remark I don't have any remarks on your excelent program.
Windy radar map provides radar data from the adjacent countries.
Slovenian meteo. service seems do not provide data
for the area beyond the red line.
Istria is not covered by Croatian and Sebian wx. radars.
The area of Istria is covered by the Croatian radar in Puntijarka
but this radar seems inactive since 23 Apr. 2020.
.... radar coverage
OK. I understand. Anyhow, some apps are using Slovenian radar data beyond the line described in your answer. I.e. app Vreme Slovenija. Obviously the data are on the outer limit of radar and therefore less reliable.
As I spend quit a lot of time on the peninsula tip and I am able to valuate the radar picture of Slovenian radar in this area, I am still using the radar pictures from Vreme Slovenija app as reliable, southern of the peninsula tip, as well.