We tried to design Hegemony so that it's fun without having to do any math, but for any inquiring minds, here's how it works:
The short answer is: skills exponentially grow, and stacking exponentially decays. If you want to see the math, keep reading, otherwise you can jump to the end.
First, find out your skill bonuses. These are the same for both the units and the generals, and they double with each level:
0, 0.125, 0.25, 0.5, 1.0
Add together the bonuses for both your unit and your general. Also add 1 if it's near a shrine, and another 1 if it's garrisoned.
Your final consumption rate is divided by (1 + consumption bonuses).
So for fully levelled units, your stacking for each of the four bonuses looks like this:
1/2, 1/3, 1/4, 1/5
For a unit which is not garrisoned, not near a shrine, and has no general, the bonus for each logistics upgrade looks like this:
8/9, 4/5, 2/3, 1/2
So although it's always useful to improve your logistics, it's a lot more useful to units in the field than it is to garrisoned units, since you're going to get diminishing returns by stacking buffs.