Here is the optimum burst altitude based on height for a desired PSI - represented by the doted line. This is based on a 1 kt (tiny) nuke explosion - suitcase size. The curves get way more dramatic as the kt gets bigger. Hiroshima was about 20 kt. The Russians have a 100 MT (100,000 times this curve) for comparrison purposes. Not sure if its still in inventory though. The Americans like 300 kt the best I think since they have more 300 kt bombs than any other type.
