There is a whole science area devoted to explosive quantity distance and related arcs. Long story short - someone has calculated the required distance for evacuation based on standard quantity-distance tables. If you have X amount of explosives in the bomb; you must evacuate non-essential personnel to this distance and fudge a little longer. The basic distances are:
Magazine distance - an explosive magazine or bomb detonates and adjacent magazines/bomb to not detonate immediately (see: sympathetic detonation). How much explosive determines needed distance. Conversely, if you have a fixed distance - that distance limits the amount of explosives that can be present. Everyone dies at that distance - strictly to prevent immediate transmission of detonations from magazine/bomb to mag/bomb.
Intraline distance - specifies the required distance between operating explosive line buildings that have dissimilar operations, i.e. explosive melt pour in one building and projectile painting in another. Protects against sympathetic detonation and limits building loss. People will be killed or seriously injured at this distance - property protection.
Public Traffic Route distance - Specifies distance from explosive locations to auto routes, trains, ship channels, etc… Provides a fair degree of personnel protection due to transitory nature of exposure to the explosive site and that a car or other conveyance has structural elements that mitigate the effects of an explosion (shock - fragments). Personnel expected to survive except for driving off the road from surprise/shock.
Inhabited Building Distance (IBD) - provides protection to personnel in the open or in structures. Then there is additional distance factor in for substantial glass exposure (more distance) and way more distance required for accidental or designed demolition operations such as the bomb defusing in this case.
For light (not) reading on this subject you can download incredibly large PDF files of DOD 6055.9 (multiple volumes) or DA Pam 385-64 from the inter-tubes.
There are computer assists now but still a lot of hand calculator work. The equations and constants have multiple exponents stretching into long decimal numbers. I did this stuff for years and it can be mind-numbing. Year-to-year change are also a headache.
Anecdote: We (US contractors and I) are destroying decaying munitions in Iraq after Desert Storm II. Based on calculations for 4 - 25,000 pound net explosive weight pits being donated; we needed 2.7 miles distance for us observers. The guys triggering the explosion just under a mile in a bunker. That worked for several weeks until a different method of stacking and priming the explosives was tried. Undetonated projectiles were thrown and landed beyond where we were standing. We moved back to 4 miles minimum from the “daily” detonations. Our experience also altered the book value equations when we reported our little mishap.