We’ve got some pretty good ability to estimate. We know how electronic waves fade over distance, we know how the atmosphere chews them up, and we know about the EMP strength of various sized bombs. We know enough to have never dedicated a serious chunk of our arsenal to be used that way, given that there was a time when we were seriously looking for ways to kill the people and leave the buildings if EMP was that cool we’d have put it in our plans.
“We know how electronic waves fade over distance, we know how the atmosphere chews them up, and we know about the EMP strength of various sized bombs.”
An EMP does not decay like a point source. It is “near field” and therefore does not follow “inverse square” physics.
The EMP strength is fairly constant from a 1kT detonation to a 100kT detonation. There is very little difference in EMP field generated (not anywhere close to the 2 orders of magnitude of yield).
“We know enough to have never dedicated a serious chunk of our arsenal to be used that way, given that there was a time when we were seriously looking for ways to kill the people and leave the buildings if EMP was that cool we’d have put it in our plans.”
Those details would be classified. That’s why you wouldn’t know about them.