Naio, you need not worry about alternator charging your Starting AGM, nor modify your vehicle's voltage regulation.
If you were to accidentally drain it dead I would recommend using a higher amperage plug in charger to get it back to full relatively quickly after a jumpstart, rather than assume the alternator will do it, because it will not, and the AGM left in the 85% charged range is not a happy AGM.
No lead acid battery is happy if less than fully charged so it should never be a question of 'should I charge it to full? ' Whether it is worth your effort is argueable. I'll skip that part of the procedure..
I modified my voltage regulation, because I deeply cycle mine and want maximum alternator contribution each time I drive for as long as i drive, and the whack job stock voltage regulator inside engine computer would prematurely drop to mid 13v range, and occassionally blast back upto 14.9v without rhyme or reason.
Most vehiclew will start with a higher sought voltage for a short eriod after engine starting, then drop to mid 13's. Most people do a quick check of voltage right after starting and assume that is the voltage always allowed/sought. Each vehicle will be different and no assumptions made about any specific vehicle with no data to back it up, or the tools to even collect it.
The drop to mid 13v range is Fine and dandy for a starting battery never discharged significantly, not great when the battery is drained deeply.
