I dunno, some mortar teams use this sequence.
"One long, one short, hopefully the third on target."
That way if you survive, and the round travels past, your high..if the round blows up in front, you need to repeat the sequence over again.
One high, one low, one mid. "hopefully your second will hit tho."
Anyhow, mortars and arty and tank cannon are all the same.
Only the angle is realy changed, but the same rule's apply.
*throws two cents*
"Edit: it should also be noted the amount of training and such some of the people have here, would make alot of ww2 pilots gunners and tanker's jaws drop. Just the basic understanding of how things work and function, and the ability to practice it over and over and over for years."