Do you remember what is bastion? Outer defences, spread forward to break opponent impetus.
SO what we do?
We know we have splash dmg
we know fort has some coverage = 3 KM
this means you can place your troops around a fort in 3 km radius and they will still get def bonus of fort.
3 km = 300 sec on foot , by inf, on home territory. 5 mins
so if you quickly count splash dmg radius o 5 km it will give you answer how close to each other units should be so attacking unit 1 will spread dmg to fort and units around it. It is something like flower but not exactly.
And as you have deducted... Bastion Def allow you to poor units into defended town without risk of attacking enemy units besieging it.
hit that like button and if thread will get 100 likes I will post something more about that.