"Low on Supply" calculation

[Crossroads]

Having read and reread the manual and the hints-and-tips document I still remain somewhat confused as how a unit becomes low on supply.

Manual, in chapter 8, seems to indicate that a unit can become low on supply regardless whether it has fired or not. There is a designers note at the end of the chapter describing possible reasons for this, such as that the unit has to conserve its ammo.

In Hints-and-tips document Jason describes how he sometimes prevents from firing artillery if he knows he will absolutely need them in a near future: "When I know an attack is going to happen, I will usually withhold using my artillery for a turn or two to ensure all my tubes have ammo."

How does the calculation work? Is it done in a different way depending on a type of a unit, or is it the same for all units?

I am playing a scenario where I really appreciate the benefit of indirect fire disrupting the attacking hordes of MG42 units :mad: , and would like to know if preserving fire has an effect to ensure the mortars can hit when I absolutely need them to. Which is on every turn, but that's another story

EDIT: First, the manual states very clearly that the supply level is calculated only once the unit has fired: At the start of each friendly turn, a supply check is made for each friendly unit that fired in the preceding turn. This check is made to maintain supply if the unit is not currently Low on Supply. But then it goes on to explain how Maintaining supply works. I guess what I am looking for here is an additional confirmation that Supply is calculated only once the unit has fired...