IN-12B Nixie Comma Current Limit
2.5 mA &times; 16 k&Omega; = 40 V drop, so the sustaining voltage is about 130 V. You could measure this to verify.

If we assume the comma has similar voltage characteristics at the lower current, then you need a 40 V drop @ 0.3 mA, or 130 k&Omega;.

Or just use your resistance substitution box (start at 1 M&Omega;) and adjust it until it looks about right.