I’m sure some engineers will baulk at this, but I remember the engineers saying, at the large heavy equipment manufacturer I worked at for 37 years, the bolt hole depth needed to be at least twice the bolt diameter to get the proper torqued stretch. Beyond that didn’t add any holding power.
I imagine that might be valid for critical connections, but I've never heard that before. I have heard that bolts need to extend three threads past a nut, which doesn't really add much to strength, IMO, but is just to make sure all the threads in the nut are fully engaged.
