I don't think this is formula. This seems to be somewhat like in regard with gutter line system.
You can check conf/battle/skill.conf
// Area of Bowling Bash chain reaction
// 0: Use official gutter line system
// 1: Gutter line system without demi gutter bug
// 2-20: Area around caster (2 = 5x5, 3 = 7x7, 4 = 9x9, ..., 20 = 41x41)
// Note: If you knock the target out of the area it will only be hit once and won't do splash damage
bowling_bash_area: 0