Ok, I’d try 680 ohm 270 ohm in series to get a small margin, and a 5k1 resistor + 5k trimmer in the divider, to put the right position near the center of the trimmer.
(270 ohm gives you at least (11.5−10.6)/270 = 3.3 mA which is 1 mA for the TL431, 1.3 mA for the circuit, and 1 mA to spare. 330 ohm could work but then it’s getting a bit tight at 11.5 V)
EDIT: 220 ohm is probably better, since current is flowing through 5k+5.1k as well, see here.