tuning voltage

Setting gain by voltage output is very vehicle-specific.  Perhaps the maufacturer gives power data with car's voltage of 14.4 as default...and your car is measuring 13 V while playing the system.  You would have to figure out the reduction in power and go from there.

You will also have to have complete acceptance that the power output of the amplifier as specified by the manufacturer is true.  What I'm trying to say is, it's easy to be off the mark if you set gain to a specific voltage derived from manufacturer's data and Ohm's Law calculations.

My advice is to set the gain by standards:  by ear, to hear the clipping.  Then (if you wish), by calculating the voltage output and measuring it, as a way of double-checking your hearing.