Amp is essentially independant of the headunit so maching gain to headunit voltage is pointless. The point of the amp is to AMPLIFY the headunits output. I have never matched gain to headunit voltage and never saw a need to.
And no the voltage change wouldn't be constant. You have a 2 or 4 gauge wire hooked directly from the battery to the amp. The wire feeding power to your headunit is probably 12 or 14 gauge and run thru a series of relays and probably only sees 10-11V on a good day, so you are always going to have more power at the amp than the headunit.
EDIT: I should add you set the gain to your subs power, not the headunit power.
