Four attrs on a model problem

gistfile1.rb

#  Given a model, self, with these attributes:
#  value           :float
#  lean_mass_value :float
#  fat_mass_value  :float
#  fat_percentage  :float
#
# User optionally inputs the last three (lean_mass, fat_mass, fat_percentage), so we can determine missing values if we have at least one of the last three in addition to the value attr. 
# Is there a better way to express this? 

def calculate_all_known_values
    if self.lean_mass_value.nil? && self.fat_mass_value.present? || self.fat_percentage.present?
      self.lean_mass_value = self.fat_mass_value.present? ? self.value - self.fat_mass_value : self.value - (self.value * self.fat_percentage)

    elsif self.fat_mass_value.nil? && self.lean_mass_value.present? || self.fat_percentage.present?
      self.fat_mass_value = self.lean_mass_value.present? ? self.value - self.lean_mass_value : self.value - (self.value * self.fat_percentage)

    elsif fat_percentage.nil? && self.lean_mass_value.present? || self.fat_mass_value.present?
      self.fat_percentage = self.lean_mass_value.present? ? (self.value - self.lean_mass_value) / self.value * 100 : self.fat_mass_value / self.value * 100
    end
  end