Wow, you guys know your stuff... thanks everyone -- each response offered a good bit of knowledge. From here I went and did quite a bit more reading on the subject... here is what I found.
While there are many small variables to take into account, I found the bulk reason why my estimation was so off initially. Battery manufacturers rate their total amp hours (AH) using 10.02 as their base voltage... Why such a seemingly obscure number, I'm not sure. What is more confusing is that the total voltage drop, as Robert mentioned earlier, is only about one full volt before the battery is at the bottom of its full depth
of discharge (DOD). So, using my battery as an example, the voltage rating of a 100% state of charge (SOC) is about 12.8. The voltage at 0% SOC is around 11.6, BUT it is recommended not to drop below a 20% SOC, or 11.8v, else the battery will suffer a serious reduction in its rated cycle life.
Ok, so realistically we are left with a one volt drop of useable headroom
before the battery must be charged, yet the low voltage of the rated AH figure is another 1.8 volts below that... so, even though the discharge rate probably isn't linear, I'm still going to make a very rough estimate that we would then need to divide the usable voltage range (1v) by the manufacturers AH voltage range (2.8v)... which leaves us with roughly .36, or just over a third... So, I estimate as a very rough guideline to take the manufacturers AH rating and divide it by 3 as a practically usable figure.
This would mean, in the example of my battery that is rated as 18AH, I should expect (18)x(.36), or roughly 6.5 amp hours.
So, how does this estimation play out in reality? Well, my motherboard has a total wattage draw of about 10 watts (very efficient), then one SSD will draw another 2w at most. I also have a spinning drive which draws about 6w, but is powered up for no more than a sixth of the time the computer is running, so we will say that adds another watt average. On top of that, I am using a step-up converter to boost voltage from 12 to 19 on the feed to the computer. Lets say that there is a little bit of loss there due to resistance, but I'm not sure how much... but I would imagine an over estimation would be to add another 2 watts to our figure... which leaves us with a total draw of about 15 watts. Well, amperage equals wattage (15) divided by voltage (19), which rounding up gives us about .8 amps. Our usable 6.5 amp hours divided by our .8 amp draw tells us that the computer should run for about 8 hours before the battery is at its bottom 'safe' limit of 11.8v, or 20% SOC.
So, do these calculations prove themselves in practice? Based on running my computer and monitoring the battery levels as it goes down a few times -- yes, it consistently looks like I will get about 7 or 8 hours if I run the battery all the way down to a 20% SOC.
Now, if you were in an emergency
and needed to have power regardless of whether you were going to destroy the life of your battery, I bet you could get the manufacturers full AH (from a battery that still holds a full charge). However, I don't see how it is a realistically helpful figure to rate batteries at an inflated figure from what they will actually provide in a long term cyclical application. This isn't a product that should have inflated figures -- leave that to amplifier manufacturers. Batteries are too critical in certain situations and could mean the difference between life and death... if someone plans on having a healthy battery providing the manufacturers rated amp hours AND holding up for the rated healthy life cycle (about 400 full discharges to 20% SOC on my AGM), they may be left stranded after depleting their battery too far a few times, or find themselves where I was wondering why I'm only getting a third of the rated amp hours... the only way to know is to if they didn't go out of their way to read the small print about how these inflated figures are generated.
I accept that there is a high chance that I may be missing something here in regards to why 10.02 volts are used as the basis for measurement... but I've become too distrustful of corporate advertising schemes to not suspect that this is yet another... even though Odyssey does put out a quality product.
Anyways, I hope this info helps somebody somewhere sometime who may find themselves in a similar situation of bewilderment.
Cheers everyone and happy sailing... can't wait to get out on the sunfish this spring (yup, I'm a lightweight).