If statement issue driving me nuts

[Anthony Smith]

Hi

I have a small project and I have been unable to get the following statement to work. Any help would great.
User inputs can either self_sale_head which is a $ value,if a $ value is not add a self.estimated_weight_hd is used to get the total weight, 
the code below should return a estimated_weight_total which can be used for the total sale price. 
All works when I add the estimated_weight_total manually. I am lost as why.

def calc_estimated_weight_total(self):
    if self.sale_head <= 0:
        amount = (self.number * self.estimated_weight_hd)
        return amount

def save(self):
    self.estimated_total_weight = self.calc_estimated_weight_total()
    super(SaleNote, self).save()