Dimming an indicator with a series resistor is easy and cheap. Predicting the intensity of an incandescent indicator bulb is difficult because the light output does not vary linearly with voltage AND the response of the human eye to light is logarithmic.

IMHO the best way to determine an acceptable indicator brightness is to connect the light to a variable supply and adjust it to an acceptable level in a dark hanger. Measure the lamp voltage and current at this point. Subtract the lamp voltage from the buss voltage to get the voltage the resistor must drop. Divide the resistor voltage by the current to get the needed resistance. Multiply the resistor voltage by the current to get the resistor power.

For example, suppose your indicator looks good at 6 volts and 0.2 amps. Your buss voltage is 14 volts so your resistor voltage is 8 volts. 8 divided by 0.2 is 16 ohms. Power is 8 times 0.2 or 1.6 watts. A 16 ohm, 2 watt resistor would work well in this case.

If you can't get a variable power supply for your test you can use a rheostat (dashboard dimmer) out of an older car. Put the rheostat in series with the lamp and adjust it as needed. Measure the voltage across it at the desired lamp intensity. Without turning it, disconnect it from the circuit and measure its resistance. This is the resistance you will need. Calculate the power from the voltage squared divided by the resistance.

Regards
Brent Regan