Which voltage should I use to calculate Amps when running an inverter on a 12v system
You have to consider the voltage and the power on the battery side, to calculate the current:

$$P_\mathrm{bat} = V_\mathrm{bat} \cdot I_\mathrm{bat} \Rightarrow I_\mathrm{bat} = \frac{P_\mathrm{bat}}{V_\mathrm{bat}}$$

To get \$P_\mathrm{bat}\$ you need to know the output power of the inverter (which is equal to your load) and the efficiency:

$$P_\mathrm{bat} = \frac{P_\mathrm{load}}{\eta_\mathrm{inv}}$$

Putting everything together leads to your suggestion "b":

$$ I_\mathrm{bat} = \frac{P_\mathrm{load}}{\eta_\mathrm{inv} \cdot V_\mathrm{bat}} = 3.43\,\mathrm{A}$$