First, I show the basis data. In addition to the case of not use telephoto lens over 100 mm for close-up. This exposure time is not reading wristwatch while doing expose. If you keep looking watch, you'll not take a photo. If you count a time completely, in advance, you try to reading a second hand of a watch while you training to count.
Spot value is variable according to kind of shells and distance strictly.
But maybe above data is usable in most cases of fireworks. If using telephoto lens to close-up, close half step. If the near position less than 400m, need 28 mm wide-angle lens. Or over about 1 km, over about 70 mm telephoto lens is handy. It's very easiest and basis way.