Good question. Your inquiry made me take another look at my maps and I realized that I made an error in my original post. The blast radius used on that map is for a 500 kiloton airburst, not 250, with the furthest ring indicating 100% probability of no harm if out in the open. I've corrected that in the original post.
To answer your question, I went through several versions of these maps trying to account for different weapon yields against various targets, driving myself crazy in the process, before I settled on using a 500kt (airburst) for every target. Although both the US and Russia still have large megaton warheads in active service, my understanding is they are reserved for hardened targets, such as Cheyenne Mountain's NORAD complex. Multi-megaton weapons are simply no longer efficient methods of destruction for non-hardened surface targets. Most active weapons in both the US and Russian arsenals range from 100kt to 500kt, so I just went with the maximum yield on that range.
If we are dealing with Chinese nuclear weapons instead of Russian, the situation gets both worse and better. It gets better because China doesn't have anywhere near the number of warheads that the US does, so the number of potential targets drops drastically. It gets worse because most Chinese weapons are still in the 1 to 5 megaton yield, creating a much larger danger zone around each target. I do not account for Chinese weapons in my EAPs because the route I've chosen from Point A to Point B is still going to be far enough away from major cities and military bases that a 5 MT hit will not affect us unless we're looking at it when it detonates, and we have a contingency for that situation.