How close to an edge/crack are bolts safe?

Strangely, there is no official documentation on how close to and edge/crack you can place climbing bolts. The intention of this article is not to create guidelines but to welcome a discussion so we all can learn and make bolting and climbing safer. Some say the bolts should be placed 20 cm from edges etc but at the same time, in practice, many bolts are placed just 5 cm from edges. In the concrete industry, one simple rule refers to the pull out cone, which is normally based on the depth in cm of the bolt times 1.5. In other words, a bolt sticking in 7 cm should be placed at least 10.5 cm from an edge.

The bolt in the picture, could be considered as an example of a dangerous bolt as it was placed on a 7 cm wide "bridge" (including a small crack) sticking up some 2 cm from the rock. Noteworthy is that the bolt was a replacement from a previous bolt to the right. " If I came across this on a route, I would worry about what else the equipper didn’t understand."(Comment from David Reeve, who has been helping UIAA as an expert in other bolting subjects.)

Here are some general questions and reflections that could be discussed in order to make climbing safer in regards to how close to edges and cracks bolts could be placed?
Why do trad climbers place cams close to edges and other cracks?
Is it safer to first bolt with a 6 mm drill and then a 10 mm drill?
Is a glue-in safer than an expansion bolt placed near an edge?
The difference on bolts with potential factor one falls and the anchor with much less impact?
The difference in between bolts placed on sandstone, limestone, gneiss and granite?
The difference in between bolts placed on vertical, overhang and roof?
The differences in between a 10 mm bolt drilled 7 cm inside the rock and a 12 mm placed 10 cm inside the rock?
What about the distance from bad rock or minor cracks?
Is the risk greater for bolts placed above, beside or below edges?

All these questions comes down to understand best practice when getting close to an edge or a crack in order to increase safety? Tricky bolting situation almost always relates to the first bolts, meaning it is better to start by bolting from a ladder instead of beginning from the top. When in doubt of the most "rock safe" bolting, in regards edges etc, and the best bolting, opt for safety. If there is no "rock safe" bolt placement, one possibility is to opt for a higher bolt and place a fixed vire or a chain.