I'll add that shooting tanks twice earns you 50 points instead of the 20 you get for disabling it's movement with the first shot. I can't tell how the helicopter bonus works, but I'm guessing it's cumulative, i.e. the first one is +10, the second is +20, etc. until your jeep is lost.
I think the +10 thru +40 is what then applies to each tank you hit at that point. After +40, the next copter you down earns you an Extra Jeep,
Pretty sure it doesn't reset when you lose your jeep. So essentially, I believe you earn an Extra Jeep for every 5th helicopter you hit.
If I'm wrong, someone should correct me.
Also, is the score visible for anyone during play? For me it is not, but is briefly when I lose a jeep.