Until you compare the voltage on the brown wire to the voltage on the battery's positive post, you don't really know that you need a regulator.
You can do it directly. Put the read meter lead on the battery's positive post, and put the black meter lead on the brown wire just before it enters the regulator. If there is more than about a volt, you are losing voltage somewhere. This makes the regulator compensate by increasing the battery voltage.
You should also compare your ground side the same way. Between the battery's negative post and the reg/rec's black wire, there should not be more than .5 volt difference, and should really be closer to 0.