When we want to check the functionality of some lightning rod installation or dimension a new one, then we usually start with geometrical-electrical model. The classic models can still be used, or we can use some newer model. That means that we assume that the descending leader progress in steps. Length of those steps is equal to the length of the last permeable distance. On the other hand, we assume that it will cause the appearance of the upward leader on some object when the distance from that object becomes smaller than the last permeable distance. Before any drawing or calculating we must determine the minimum value of lightning current from which we are trying to save the object. It is because lightning that brings less electrical charge causes smaller current and progress in smaller steps. The last permeable distance for such lightning is smaller and it is the worse case for designing protection. By picking the minimum value of current we decide on quality of protection, because a lightning with smaller current could go around the lightning rod and hit the object. We are trying to determine the probability of that kind of hit and make it as small as possible. The recent researches and practice gave us the next table which shows the smallest last permeable distance for certain percentage of all lightning strikes.

It means that if we use the last permeable distance 20 m long, we will protect the object from being hit by lightning in 95% cases and current greater then 3.9 kA certainly will not struck the object. Although, there is still a small possibility that abject can be hit by lightning with current smaller than 3.9 kA.
After that, we choose the type of protecting installation and its basic dimensions. Using geometrical construction, we check if the dimensions are appropriate. If they are not appropriate, we change dimensions or shape of the installation and check again. Procedure of checking the dimensions of protecting installation is shown on this animation.