If I could offer a 'newbie" view ...
For me it is the look of the thing, so I look to rules that are not prescriptive regarding basing and then base according to appearance.
One thing that has struck me is how fashion influences how units are presented. Once upon a time 'proper' wargames figures in 'proper' scales depicted mainly marching or advancing poses. This favoured very tightly packed columns; on looking through my copy of Charles Grant's book, it is very noticeable that there were a lot of figures
very tightly packed into his battalions.
For years those using 'toy' figures from 'toy' scales had to throw out most of each packet in order to rank-up battalions. More recently, thanks to HaT and, now, Strelets, this is changing. So, I can rank-up figures a la Grant.
But, wait, what is this? Now the fashionable way to base Napoleonics seems to be almost open order, and with figures doing a variety of different things on a battalion base. I first noticed this with 28mm figures, but now it has spread to 1/72nd. So we have a figure standing smartly to attention, another in motion, on the march, a third standing to load his musket, a fourth firing his and a fifth charging the enemy, and so forth.
I am evidently old fashioned. I like battalions ranked up and all doing the same thing. That 'same thing' is preferably 'march-attack', but advancing or standing are also fine.
But, I do like to get a reasonable view of the figures, which becomes difficult if they are wedged too close to one another.
So I took to trial and error with compromise in view. I have found that, for me, the best impression of numbers, whilst still allowing a good look at the figures yet giving the impression of a compact and orderly body of men has been to base each figure centred on an area 16mm wide and 17mm deep. These are then mounted semi-permanently on 2-deep company bases, surrounded by a border that creates an added gap between the companies.
Having based a regiment in this way -
http://bennosfiguresforum.com/viewtopic.php?f=2&t=20251 - I am comfortable that the approach works for me, but, of course, it is a subjective choice.
So, really, I have ended up taking the opposite approach to the OP; I have decided on how many figures I want and the base dimensions are then dictated by the figure spacings I have adopted because I happen to think they look OK!
This is probably entirely the wrong way to go about things, and it is certainly not scientific, but I throw it into the mix for what it's worth!