love_letter_calculation_effect

love_letter_calculation_effect = {
	hidden_effect = {
		save_scope_as = letterAuthor
		duel = {
			skill = diplomacy
			value = 10
			25 = {
				compare_modifier = {
					value = scope:duel_value
				}
				set_variable = {
					name = love_letter_quality
					value = 4
				}
			}
			25 = {
				compare_modifier = {
					value = scope:duel_value
				}
				set_variable = {
					name = love_letter_quality
					value = 3
				}
			}
			25 = {
				set_variable = {
					name = love_letter_quality
					value = 2
				}
			}
			25 = {
				compare_modifier = {
					value = scope:duel_value
					multiplier = -0.5
				}
				set_variable = {
					name = love_letter_quality
					value = 1
				}
			}
		}
	}
}

As written in game/common/scripted_effects/00_romance_effects.txt.