Those kinds of charts are maid for different areas. On the isocheraunic chart of world we can see that the areas around equator have more lightning strike days than on the far north or south where that number is almost zero.

Scientists have suggested few expressions that give us average number of lightning strike days in 1 square kilometre per year. One of them is:

In this formula Ns is expected number of lightning strike in 1 square mile per year. Ni is isocheraunic level (number of lightning strike days per year) and alpha is geographic width of area. We can see that the results can variate and could be used for orientation. Today some developed countries use devices which record lightning strikes in some area. Networks of that devices give us more precise informations about lightning strikes per year in some areas.

To get probability of lightning strike in some object we need the information about ground surface on Earth where the lightning will strike. That information we can get from the geometry of desired object. How?
We imagine the model of that object and the sphere, which radius matches final permeable distance for the average lightning strike current in that area. If we role the sphere around that object and it touches it. We record places where sphere touches ground and we got the wanted surface.
When we know the average number of lightning strike per year on 1 square kilometre (Ns) and we he have wanted surface (A), then we divide Ns with A and we get the average number of lightning strike in some object per year.