An air blast wave having 20 psi overpressure is enough to destroy most of the concrete structures, and cause almost 100% fatalities in its way (but causing negligible fallout). A 200kT warhead aimed for that sort of destruction should be detonated at around 1000 metres altitude (1060m to be precise).
If the altitude is lesser than that, the blast wave will have higher pressure but lesser effective radius. And if it is greater than that, the pressure will be reduced while effective radius would increase.
You should try this calculator:
NUKEMAP by Alex Wellerstein