the ticks, i believe, are a measure of distance in yards( eg 18 = 1.8 d) However this isnt the end of the story, different elevations from shooter to target must be considered as well as recoil.
as far hittin other base targets, no secrets here bud, practice is the key. You can lob shells in and then guesstimate your correction from the crater to the intended target.
and yes all hangers, factories, fuel tanks... can be killed. HP shells work best for these though you sacfrifice range.