Lightning Vs Splash Towers and Air Defense

I get using lightning to take out a single mortar or wizard tower (2 spells) or air defense (3 spells) but how do you go about killing two at once? How do you calculate how much damage a tower will receive if you drop lightning right on the edge of its 3x3 space rather than in the center?

More importantly, if two L4 air defenses are touching at the corner, is it possible to destroy both with L3 lightning? What about L4? If it's not possible with either, how would you maximize the damage on one while ensuring the destruction of the other? Could you drop two directly on it and then one right on the point where the corners of the two air defenses' spaces touch?

Confession, I have failed an all-drags attack because I failed to completely destroy an AD with 3 lvl5 lightnings, so I make sure now to drop them in the center.

For mortars though I drop one in the center and another on the line between it and whatever building is next to it (hopefully another splash) to maximize the damage on the other building since one lightning depletes more than 2/3 of the mortars HP.

I am not sure about Air Defense, but three lvl 4 light spells will kill two mortars sitting next two each other. I do it all the time. Even better if you can lure the clan castle troops on top of the mortar! The key is to zoom in and take your time!