Air Danger Areas are drawn in accordance with a 'template' which is produced from guidance material in 'Pam 21' ( an MOD 'restricted' document) and as such, allowance is made for possible errors eg negligent discharges (yer squaddie letting one off accidentally) and ricochet hazard for small arms ranges, and for air ranges eg bombs/rockets, for 'forward throw' of the weapon. When I visited Holbeach, they told us A-10s had to use a different target to other ground attack aircraft as otherwise, the template would have infringed the built up area of Boston (Lincs not Mass!!)