1.) In my chart, from 100-600, my mil readings for elevation are all minus?
2.) which of these is the closest?
(a)Army 3.375 MOA
(b)USMC 3.438 MOA
3.) How do I choose between G1 or G7?
1.) This sounds like you've entered a distance further than 600 yards as your zero distance... so everything short of that distance will have a negative value, meaning you'll have dial back your scope from your zero distance.
2.) Your scope uses the USMC standard of 3.438MOA... I determined this by going to the techinical page of your reticle (http://www.premierreticles.com/pdfs/2009-5-25xxGen2XR.pdf) and looking at the value of 1MOA - which in this case equals 3.6 inches at 100 yards, which is equal to 3.438MOA.
3.) This question is a good one and one that I get fairly often (I'm working on a video tutorial for this subject as well). Unfortunately it's difficult to answer quickly, as there is a bit information that you (and others) should know first... Keep in mind, this is not an exhaustive answer - the subject of drag models is a little more complex than I describe below.
Most ammo manufacturers publish their ballistic data using the G1 drag model. There exists several drag models though (G1, G2, G5, G6, G7, G8), each describes a different shaped bullet - G1 being the 'standard' comparison shape. The odd thing is though, the G1 bullet looks more like a
round-nose .22 bullet than todays more advanced VLD boat-tail rounds, so ammo manufacturers have to convert their ballistic data, from whatever drag model that shape bullet uses, to the G1 drag model.
They do this because the equation for the G1 drag model gives a higher Ballistic Coefficient value... this does not mean that one Ballistic Coefficient is necessarily better than another, as a proper G1 or G7 ballistic coefficient will give you similar trajectories. What
can be said, is that using a bullets native ballistic coefficient (meaning, if a bullet is shaped like a G5, use a G5 BC - if it's shaped like a G7, use a G7) will result in a more accurate end result, particularly out to longer ranges (800 yards +).
Here is a quick example: A bullet manufacturer has published a G1 BC of 0.659 for their 7 mm Match Target VLD bullet. The same bullet has a G7 BC of 0.337. For the untrained person, the G1 "looks" to be a better number because of higher numerical value and thus, manufacturers choose to publish G1 numbers. The irony here is that using a proper G7 B.C. will result in a more accurate result for the shooter, the challenge is finding these numbers published. An excellent resource, if you're interested to know more, is the book
Applied Ballistics for Long Range Shooting by Bryan Litz - Not only does his book do a great job of bringing this complex subject matter into easily understandable terms, but it also includes 175, G7 ballistic coefficients that Bryan personally field tested and calculated using a very precise measurement system... Definitely worth the read and purchase, if only for the B.Cs.
Additionally, I always recommend that users measure their ammo's velocity at two, or more, known distances (0 and 100 yards, for example) and then use these numbers to find an accurate B.C. for the average speed out of your rifle. This requires a Chronograph though, which is not something that all shooters have or want to purchase; Using the published G1 B.C. in this case will, at very least, get you on paper.
The short answer to your question is that the vast majority of people will continue to use the standard G1 drag model, but there are some bullets in the iSnipe database, as well as additional factory ammo available, that use a G7 B.C.
