Was about to get a toy- then I did some research...

[Juranovich]

Above should explain this hope fully.! . . . . Don't try to overthink this, I understand the need to understand how it works but if you want to build a good machine then what's been suggested will work great. You could spend weeks or months learning how it all works and you'll still end up back at what's been suggested.
Go with what's proven to work and you won't go wrong.

This! Even though I'll probably end up building my machine according to the various suggestions I've got, I still cannot bring myself to do something without having built an understanding of my own of the matter. Hence, the occasional silly questions :)

[Juranovich]

Moving on to the mechanical side of things (at least for now) and essentially the gantry arm design. Am I right in assuming that it is the centre of gravity of the entire gantry assembly (incl. gantry arms, beam, z-axis, spindle etc.) that preferably should be half way between the X-axis (long axis) rail carriages?

My thinking is that any force applied to the spindle in the X direction affects the balance of the entire gantry assembly. Now, the reason I'm double checking this is that when drawing the gantry assembly (as in the attached, albeit, unfinished drawing) I find that the COG is quite far back (the left side mark) if the beam is also included in the calculations. I know some parts are still missing from the drawing, but even when included, the end result will not change much (in fact, what's missing is mostly parts on the left hand side of the mark). Bottom line is, in this scenario my gantry arms would raise straight up from the X-axis carriages (hence, neither would the inclusion of the arms affect the cog) and, essentially, leaving the spindle quite far in front of the front carriage.

Problem is that nearly all gantry arm designs I've looked at will have the spindle closer to the halfway point between the carriages or at least somewhere between the carriages. Now, when I calculate for the Z-axis assembly only (the mark to the right) the COG is naturally much closer to the spindle, and should I use this as my reference point my gantry arm design and more importantly the spindle position (relative to the X carriages) would end up looking much more like what I see others using. My intuition still says I should go with the COG of the entire gantry assembly as my reference point, but seeing that this would end up looking much different to the norm, it makes me wonder if I've overlooked something or simply using the wrong logic?

[JAZZCNC]

Moving on to the mechanical side of things (at least for now) and essentially the gantry arm design. Am I right in assuming that it is the centre of gravity of the entire gantry assembly (incl. gantry arms, beam, z-axis, spindle etc.) that preferably should be half way between the X-axis (long axis) rail carriages?

Yes Ideally but again without wanting to sound like a broken record don't let these kinds of details bog you down from the building phase. The COG being off a little like what your showing isn't going to make one jot of difference to how the machine performs or how it affects component life in a DIY environment.

Anyone who's built a machine and worried about these kinds of things will tell you that it was a pointless exercise and it's the smaller details that make a bigger difference to how the machine performs. Details like sturdy ball-nut brackets and adjustability, motor mounts, wire routing, Limit SW placement, access to grease nipples and lubing, etc, etc.
If you build a Sturdy structure and pay attention to key areas like Z-axis then you won't go wrong or notice if COG is off a little, but you will notice if the little details are missed.

My advice is to look around at other builds and pay more attention to the little details and pay less attention to if COG looks a little offset.!

[JAZZCNC]

I still cannot bring myself to do something without having built an understanding of my own of the matter. Hence, the occasional silly questions :)

The only silly question is the one you don't ask...Keep firing away we understand.!
What's I'm trying to say more than anything is don't let it slow you down from building because you'll still end up back at or around what's been suggested.