Work out cron schedule next run instantly with clear inputs, formula shown and shareable results.
A cron expression like 5/15 fires at fixed minutes past each hour, so the next run is the first scheduled minute after now. With a five minute offset and a 15 minute interval the fire times are 5, 20, 35 and 50, so at 37 minutes past the next run is at 50 - thirteen minutes away. An interval that does not divide 60 produces an uneven gap at the hour boundary.
Next scheduled minute
next = smallest offset + k x interval greater than the current minute
Frequency
runs per hour = 60 / interval
Because cron restarts the pattern each hour. */25 fires at 0, 25 and 50, then again at 0 - a 10 minute gap rather than 25.
Use a scheduler that supports intervals rather than cron fields, or express the schedule explicitly minute by minute.