diff --git a/sale_invoice_plan/models/__init__.py b/sale_invoice_plan/models/__init__.py
index 669fdb833cb..7f7ce410698 100644
--- a/sale_invoice_plan/models/__init__.py
+++ b/sale_invoice_plan/models/__init__.py
@@ -1,4 +1,5 @@
 # Copyright 2019 Ecosoft Co., Ltd (http://ecosoft.co.th/)
 # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html)
 from . import sale
+from . import sale_order_line
 from . import sale_invoice_plan
diff --git a/sale_invoice_plan/models/sale_invoice_plan.py b/sale_invoice_plan/models/sale_invoice_plan.py
index b61496d3023..b838547b964 100644
--- a/sale_invoice_plan/models/sale_invoice_plan.py
+++ b/sale_invoice_plan/models/sale_invoice_plan.py
@@ -93,22 +93,29 @@ def _compute_no_edit(self):
 
     @api.depends("percent")
     def _compute_amount(self):
+        """Amount to be invoiced"""
+        Decimal = self.env["decimal.precision"]
+        Decimal.precision_get("Product Unit of Measure")
         for rec in self:
-            amount_untaxed = rec.sale_id._origin.amount_untaxed
+            amount_untaxed = rec.sale_id.amount_untaxed
             # With invoice already created, no recompute
             if rec.invoiced:
                 rec.amount = rec.amount_invoiced
-                rec.percent = rec.amount / amount_untaxed * 100
-                continue
             # For last line, amount is the left over
-            if rec.last:
+            elif rec.last:
                 installments = rec.sale_id.invoice_plan_ids.filtered(
                     lambda l: l.invoice_type == "installment"
                 )
                 prev_amount = sum((installments - rec).mapped("amount"))
                 rec.amount = amount_untaxed - prev_amount
-                continue
-            rec.amount = rec.percent * amount_untaxed / 100
+            else:
+                # Simulate the amount to be invoiced, as close as possible
+                # For this, round quantities on each order line
+                rec.amount = sum(
+                    sol._get_installment_invoice_amount(rec.percent)
+                    for sol in rec.sale_id.order_line
+                )
+            rec.percent = rec.amount / amount_untaxed * 100
 
     @api.onchange("amount", "percent")
     def _inverse_amount(self):
diff --git a/sale_invoice_plan/models/sale_order_line.py b/sale_invoice_plan/models/sale_order_line.py
new file mode 100644
index 00000000000..63905591176
--- /dev/null
+++ b/sale_invoice_plan/models/sale_order_line.py
@@ -0,0 +1,16 @@
+# Copyright 2023 Open Source Integrators
+# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html)
+
+from odoo import models
+from odoo.tools.float_utils import float_round
+
+
+class SaleOrderLine(models.Model):
+    _inherit = "sale.order.line"
+
+    def _get_installment_invoice_amount(self, percent):
+        """Simulate the invoice lime amount, after applying a percentage to it"""
+        prec = self.env["decimal.precision"].precision_get("Product Unit of Measure")
+        quantity = float_round(self.product_uom_qty * percent / 100, prec)
+        invoice_amount = self.currency_id.round(self.price_unit * quantity)
+        return invoice_amount