Pretty much the same with mortar-fire . . .
We had a nifty thing called TOT, or time-on-target. Since the guns are stationary at the time of firing, estmating a convoy's velocity is relatively easy. You prep a known point, perhaps a tree or an intersection, so the actual target is already solved. It's just a matter of having your bullets arive at the right time. That's when you learn to love or hate your forward observer.