In OpenERP first, we will manage all file and create module and then inherits to the account.invoice object in your .py file.

Follow these step given below

Step1- First, we create custom module like as a test module and put this test module in your server and then install in your database.

Step2- Then create .py file in your test module and file name like: test.py file and pass test.py file in your __init__.py file. After this writes function and fields here and same in database.

Using this code given below:

def onchange_squences(self, cr, uid, ids, qty_char, product  , price_unit=0, discount=0, quantity=0, context=None):
        qty_char = qty_char
        product_obj = self.pool.get('product.product')
        product_obj = product_obj.browse(cr, uid, product, context=context)
        prod_type = product_obj.supp_ref
        squ_meter = product_obj.squ_meter

        price = price_unit
        value = {}
        if prod_type == 'T':
            if qty_char:
                typ = float(qty_char)
                qty = typ / squ_meter
                round = math.ceil(qty)
                new_qty = round * squ_meter
                value['quantity'] = round
            subtotal = (quantity) * (discount)
            value['price_subtotal'] = subtotal
        elif prod_type == 'B':
            if qty_char:
                typ = float(qty_char)
                qty = typ / squ_meter
                round = math.ceil(qty)
                new_qty = round * squ_meter
                value['quantity'] = round
            subtotal = (quantity) * (discount)
            value['price_subtotal'] = subtotal
        else:
            if qty_char:
                typ= float(qty_char)
                value['quantity'] = typ
            subtotal = (quantity) * (discount)
            value['price_subtotal'] = subtotal
            
        return {'value': value}