This is probably the formula for bombardment chance&damage
AP = Bombardment Ability Point
XP = Experience level

Default
Chance: Sum(AP * 0.015 + EXP * 0.02) + 0.1
Damage: Sum(AP * 0.01 + EXP * 0.01) + 0.02

Modded
Chance: Sum(AP * 0.01 + EXP * 0.01) + 0.1
Damage: Sum(AP * 0.0066 + EXP * 0.01) + 0.02

Ex: A single Frigate (Kaiyo Maru) has 30 AP, meaning by default it'd have 55% chance to inflict 32% damage. With my tweaks, it now has a 40% chance to inflict ~22% damage. Some caveats: it's not apparent how damage would spread in a city bombardment, or whether missed shots can still inflict lower amounts of damage.