Cron every hour at 30 minutes past: 30 * * * *

Changing the minute from 0 to 30 does not change the frequency at all, only the phase. The job still runs 24 times a day, still once per hour, and still every day of the year. What it stops doing is competing with the pile of work that every system schedules on the hour.

Half past is the natural choice when the hourly job depends on something else that runs on the hour. If a metrics flush happens at hh:00 and takes a couple of minutes, a consumer at hh:30 reads a settled snapshot instead of racing the writer. The dependency is implicit and fragile, but it is far better than both jobs starting in the same second.

Any minute from 0 to 59 works, so if you run several hourly jobs, give each one a different offset rather than putting them all at 30. Spreading four hourly jobs across minutes 5, 20, 35 and 50 turns four simultaneous spikes into four small ones, with no change to what any of them does.

The offset does not survive being converted to a macro. @hourly is defined as 0 * * * * and there is no way to express half past with it, so a crontab that mixes @hourly and 30 * * * * is describing two genuinely different schedules even though people often talk about both as hourly.