This article has a great description on how to do a safety hole, it's about 1/4 of the way down.
https://www.backyardchickens.com/articles/guide-to-assisted-hatching-for-all-poultry.72886/
https://www.backyardchickens.com/articles/guide-to-assisted-hatching-for-all-poultry.72886/