Depends on the specific situation, really. However, the key thing to note is that unless your enemy is very small and has only one city very close to your borders, you are going to need to build new logistical bases further in as you advance - it's rare for preparations beforehand to be wholly adequate to the task of conquest, especially of a major regime.
Other general notes:
1) Supply depots can expand your range, but not your capacity. If you're going to push additional brigades to the front during the course of the war, you'll need more truck stations eventually.
2) You can usually survive for a little while at the far end of your supply lines - this can give you the breathing room necessary to establish new forward logistical bases, so don't be afraid to push past your logistics a little if it gives you an edge.
3) For absolute best practices against a serious major regime, you'll usually want a rail line extending from your SHQ to the closest point to the front terminating in both a railhead/station and a truck stop. Generally speaking rail should provide the main arteries of supply, with truck stops providing the capillaries. This isn't as important early on when you're fighting your first war with a minor regime and it is expensive to set up, so early wars can be fought with truck stations but eventually you want to link all your cities by rail, with rail lines to all major fronts.
4) That being said, once the offensive kicks off it'll probably be too expensive and time-consuming to set up rail into conquered territory on a regular basis, so you'll want to use truck stations and supply depots as your offensive arm, occasionally establishing new rail lines to consolidate your gains once you have enough to be worth consolidating.
5) Remember that shipping replacement troops from SHQ to make good your casualties costs logistical capacity above and beyond normal supply purposes - always make sure you have a healthy logistical surplus to cover that. It'd also help if your logistical network was strong enough to allow strategic transfer of brigades as well, to help provide you with flexibility if things aren't going well.
6) When positioning your supply assets, pay close attention to your logistical overlays - you want to position your new assets right where they can pick up the slack of the old assets when they start faltering. Too far out and your supply lines will be weaker - too close in and they'll be inefficient. A second station five hexes out sounds pretty close in, to be honest, but check our logistical overlays to be sure. When conducting an offensive, it's usually best to have a logistical asset as close to the border of the attack as you can, to maximize the amount of time and territory you can go through before needing to construct a new one.
7) I mentioned this in the other points but taking a second look at your posts I feel the need to stress this: Make use of your logistical overlays - current logistical points, initial logistical points, used points, etc. Build your logistics on the basis of hard data, not what's realistic or feels right.
8) Finally, sometimes in order to learn how things you work you need to just bite the bullet and kick things off and see what happens. Don't be afraid to experiment - or even lose. Learn what you can from each failure and keep diagnosing your weaknesses.