field replace on odoo using xpath

I've tried Odoo 9 and want to replace the "Total" field = amount_total with a new field which is the result of some addition of field content (sum_of_total) with XPath expression but it's failing and not returning any result.
Here is my XML code :

<openerp>
    <data>
        <record id="purchase_order_form_training1" model="ir.ui.view">
            <field name="name">purchase.order.form1</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
                <field name="amount_tax" position="after">
                    <field name="down_payment" widget="monetary" options="{'currency_field':'currency_id'}"/>
                </field>
                <xpath expr="form1/sheet/page/group/field[@name='amount_total']" position="replace">
                    <field name="sum_of_total"/>
                </xpath>
            </field>
        </record>
    </data>
</openerp>

The odoo log didn't show me any syntactical error. Can anyone help me to identify the root cause?

Refer below to understand how sum_of_total is getting initialized

from openerp import models, fields, api, _

class purchase_order(models.Model):
    _inherit = "purchase.order"

    @api.one
    @api.depends('down_payment', 'amount_total')
    def get_total_after_dp(self):
        dp = self.down_payment
        tot = selt.amount_total
        tota = tot + dp
        self.sum_of_total = tota

    sum_of_total = fields.Float('Total Amount',
                                    compute='get_total_after_dp')

    down_payment = fields.Float('Down Payment')

There is no form1 in the purchase order form view. try this:

<xpath expr="form/sheet/notebook/page/group/field[@name='amount_total']" position="replace">
     <field name="sum_of_total"/>
</xpath>