Hi Greenone,
You bring up a very common question and one that continues coming up for considerable debate.
I suggest setting your D8 around 4.5-5 on the dial (necessary to prevent the input voltage issue that results in popping on one channel), and setting your V2 as needed to see the levels you would like. I would expect 30dB give or take 5dB depending on source level. This should give you levels in the -8 to -2dBfs range based on typical concert levels of 100dB at front of house.
I base this feedback on the results of the major manufacturers of pre-amps and A/D converters.
My belief is that you are best off running your pre-amp hot enough to achieve a level that will provide optimal dynamic range.
Grace, like Apogee and Benchmark, have all optimized their gain structure using Audio Precision System One and Two measurement systems (industry standard in the audio world) and have each respectively determined that the best signal-to-noise ratio (and optimal dynamic range), that is the BEST performance from their units, is achieved by maximizing level from the pre-amp (usually around +30dB of gain for loud music sources (around 100dB, typical of concert levels at FOH) and calibrating the A/D as necessary to achieve full scale zero with the pre-amp near its maximum output. This is why Grace requires +25dBu for full scale zero, Benchmark will accommodate a +14dBu level but recommends using a pre-amp that can deliver +20dBu or greater, and Apogee factory calibration is set at +20dBu=0dBfs for line level input.
I continue to find this interesting as I know a lot of people are running analog-to-digital converters (or decks for that matter) “wide open” and only using a bit of gain from the pre-amp (ex: V2 at 10 or 15dB of gain with a AD-1000 “wide open”). Conceptually, this would follow your thought of not adding gain only to pad it later, but the research above demonstrates something significantly different.
I know there is information to the contrary out there regarding gain structure but the manufacturers I respect most, all adhere to the above design approach.
I'd love to hear about real a/b comparisons made contrasting these approaches and any comments about whether they made an audible difference or not.
As for any lead-acid battery, they don't have memory issues and should be charged after each use (as John already said). This will dramatically improve the life expectancy of your battery.
Happy Recording Everybody,
Marc