Gain sensitivity questions

I typically do all my own audio work but theres a couple things I'm not quite certain on that maybe others can clarify!

1. When using test tones and ac voltage to set gains, is it better to use 0db or -3db tones (50hz subs, 1khz mids)? Ive heard that the 0db tones leave power on the table and dont really fit real world volumes potential as well?

2. When setting gains for amplifiers with their max output @ 14.v, how do you descale and calculate when your actually setting it in the boat with 12.6v at the amp? I usually calculate the percentage of voltage difference and use that percentage transfer to the ac voltage. Does that make sense?
14.4v/12.6 87.5% then times the original AC voltage for rated watts but I'm not certain that amplifier is linear in output through the voltage range?

1) 1kHz on fullrange or highpass zones and 50 Hz on the lowpass. 0 dB might be the choice if you listen to mostly recent recordings with maximum compression engineered to sound louder or music with max compression, very dense material and little white space (perhaps Nickleback). 0 dB is the maximum peak that can be found on the CD or digital source. I listen to material from older recordings with far greater dynamic range so I would use the - 3dB or even - 5dB level. You can't play a sine wave test tone through speakers at that level without damage and the measurement doesn't reveal speaker compression and the measurement doesn't balance zones, so the measurement is only to determine the maximum electronics thresholds of individual channels. The final tuning phase will always be by ear.
2) The ratio doesn't work that way. Some strictly regulated amplifiers will deliver the full 14.4V power with a 12.6V supply. Most unregulated amplifiers will have a 25% drop-off in power, some a little more, some a little less.

David, let me be a little more specific. I'm speaking of unregulated amplifiers and in this scenario a wet sounds Sd6 tuned with a 0db 1khz tone.

If bridged to a 4ohm load it assumes 585 watts @ 14.4v which would equall out to 48.37 AC volts (if everything was at 14.4...which it is not). We also know that most boats only get to about 13.8v and many listen to it when its off a 12.6 so it is unclear how to properly set ac voltage with indirect measurements and not over/underpower a speaker.

1. When setting for ac voltage output, does it matter what the input voltage is at the time of setting (ie. Do i need to have the boat idling to set when it reading 13.8)

2. Should i estimate ac voltage to 13.8 and set roughly since its closer to the mark of 14.4?

The reason for the question...not only to be more accurate with all my settings but is to set proper sensitivity for my rev 10's with my sd6 bridged to get max output without overdoing it. And yes I am very strict with people not messing with my stereo as well as myself not going over a certain setting when playing.

585 watts is a lot of power per each Rev10. As an example, for a 10-inch subwoofer to truly handle 600 watts RMS it usually has a larger voice coil than the Rev10 midbass driver. However, for me, it represents zero risk because I have zero tolerance for compression and I know exactly what the first signs sound like. I would certainly enjoy every watt of that peak & instantaneous power given my choice of music and yet I would never approach the continuous thermal limits. In pursuit of musicality, I want the max peak power which isn't particularly threatening to the speakers thermal capacity. Music power is different from a sine wave test tone and your music differs from mine. The final phase of the very best sounding system has to be performed by ear and by someone well initiated with an analytical ear. The voltage method only establishes a maximum and safe guideline. But you can still screw up a perfectly tuned system with bad material of bad origin or the misuse of equalization. I'm trying to point out that there are no guarantees you will get perfection with a test disc and a multimeter.
But here is as close as you can get. The 'safest' method would be...with the boat at rest in the water, with fully topped off batteries, take your rpms to 2.5K and set the gains zone by zone using a 0 dB signal at the 14.4V power rating voltage. That is as high as the amplifiers will ever produce. The 0 dB signal should compensate for any deviation.
A hand held scope or distortion detector would be more accurate for finding true clipping at a particular voltage. But again, music is different so the final fine-tune is done with real music by ear.
I know that Odin with Earmark Marine rents his full system customers a distortion detector at no charge, plus walks them through a final tuning process that is extremely sophisticated as he has a ton of pro sound engineering experience, is a musician, and understands gain structure better than anyone I've talked to.

Thank you david, i appreciate your technical input and experience. As you have mentioned, boats never see 14.4 and most of my playing is done near 12.6v which at a 25% loss from non regulated voltage puts me near 435 actual watts while playing and maybe 500 watts while driving. Most of the time I'm at a safe 75 percent volume so i dont anticipate I'll have many issues unless i come accross a really hot recording.

This may be a stupid question, but i think it pertains to most of us tuning. When running music/tones from your phone (since most of us do these days) should we use full volume on the source (phone) while playing or does the signal get clipped at full phone volume like a cd player and must be turned down a notch or two? Audibly I havent noticed a difference on my Galaxy S5 but I previously setup volumes with my phone and ws420 @ 75% to be safe but I'm not sure its necessary.

You cannot correctly tune a system without finding the true clipping threshold of every single component in the audio chain from beginning to end. Many source units do not clip at all at full volume. Only a rare few would clip as early as 75%. You want to get the maximum voltage from every piece which ensures the lowest noise floor in combination with the highest dynamic range.
Ideally I would want the identical voltage and gain level from my phone as with my CD player which might require a small line driver on the phone.
Heck, Odin with Earmark Marine wrote a draft of over 2000 words on the process of correctly tuning just an EQ from a pro perspective. That's nothing as compared to a full system process. Then he can get into some advanced concepts for those with the tolerance for something real technical.
You can get some very primitive guidelines on a forum. But nothing terribly precise. There just aren't the quick short cuts that you are hoping for....not if you want to do it right.

Success rarely comes from taking the easy road, so I'm not looking for short cuts but more clarity of details. Thank you for providing those.