Straight across overhead is a tough shot because of the twisting body for the follow thru. Best to shoot when they are still out a bit further at say 10:30 to 11 oclock position. Allow plenty of lead too depending of speed and distance.
Straight at you is an easy shot, just point the guy at them and shoot a bit high-and duck to be missed by the falling birds. Going away you lead below them a bit.
Crossing shot to the side is a good shot in that you should have good body position. The main thing is to lead enough and follow thru.
Ducks climbing, you need the shoot above them for the lead.
There are 2 methods to leading a bird. 1st is sustained lead and squeezing the trigger. 2nd is put the bead on the bird and swing out in front of the bird and then squeeze the trigger. Also--you squeeze not jerk the trigger becuase if you jerk(flinch too) you will be way off target.
As for the amount of lead-good question. There are a lot of variables. the ducks speed and the FPS of the loads you are using, type of shot. For example steel HV shot of initiially very fast but slows down much quicker at about 35 yds and you will need more lead at 45 yds compared to traditional lead shot or Hevishot. Sorry but I do not have a table of leads required for the various loads and FPS and ducks speed. I have heard some say at 30 yds you lead the width of your barrel. I just instinct shoot and do well to 35 yds, further than that and I do not lead enough. The difference between a rifle and a shotgun is this-with a rifle you aim at the target, with a shotgun you point the gun, get my drift?
For many hunters lead is automatic. Eye/hand coordination thing. It is like passing a football to the receiver--you just automatically (instinct)lead them. Bottom line is nothing is better than experience. Practicing at the skeet range is best way to hone the lead skills in IMHO. Frankly some guys do not pick up a gun for 9 months and then wonder why they shoot poorly. Thus why a few trips to the skeet range in the summer is advisable to polich the skills. But if you lack the skills, then going every week over the summer is best way to learn to shoot well IMO.
You may also want to check out the reloading/ballistic forum to see if anything is posted up there.