We recently released a new feature in our application: Predicting cron jobs' run times.
As you can see in our example page, once your added a cron job and enable the "Logs and Predictions", you will see latest logs in the past and run time predictions in the nearest future for the cron job (Log and prediction number depend on the plan quota. You can also configure them in the cron job "Logs and Predictions" panel).
We believe that by seeing when will your cron job run, you will be more sure how to set the cron job correctly.
Just play it around and let us know your suggestion if you have any.