Thanks for the solution. I've tried and it is working perfectly.

Hi,

Try with the following code,

@api.depends('standard_price', 'margin')
def _compute_list_price(self):
    for record in self:
        if record.margin:
            record.list_price = record.standard_price * (1 + record.margin / 100)
        else:
            record.list_price = record.standard_priceH

Hope it helps

Hii,
Here is updated code 

from odoo import models, fields, api

class Phone(models.Model):

    _inherit = "product.template"

    margin = fields.Float(string="Desired Profit Margin (%)")

    list_price = fields.Float(

        string="Sale Price",

        compute="_compute_list_price",

        store=True  

    )

    @api.depends('standard_price', 'margin')

    def _compute_list_price(self):

        for record in self:

            if record.standard_price and record.margin:

                record.list_price = record.standard_price * (1 + record.margin / 100)

            else:

                record.list_price = record.standard_price or 0.0  # fallback to cost or 0

    @api.onchange('margin', 'standard_price')

    def _onchange_margin(self):

        for record in self:

            if record.standard_price and record.margin:

                record.list_price = record.standard_price * (1 + record.margin / 100)

i hope it is use full