For my first try, the output was about 30V! Far more than the attended 12V.

It appears that I am not using a LT1302 but a LT1302-5 for which some R4 and R5 are internally mounted. So, if you are using a LT1302-5 too, remove R4 and use a 570k resistor for R5.

(There is an intern 315k resistor so 315k + 570k ≈ 866k.)