It’s really not impossible to calculate, particularly if you’re trying to cause damage.

The answer is it’s cheaper to shoot down individual satellites than try to create a localized cascade. Kessler cascades propagate too slowly, and degrade too quickly in low orbits, to be useful as a military tactic. In high orbit one could feasibly e.g. deny use of a geostationary band. But again, it’s cheaper to just shoot down each satellite.