Which trigger to implement this?

I am trying to write code on Sales Order's subform;
Quantity Remaining = Quantity - Quantity Shipped - Reserved Quantity

Quantity Remaining is a new field I added by extending the Sales Line table and Sales Order Subform.

This value will be Quantity - Quantity Shipped - Reserved Quantity
However, Quantity Shipped only gets a value once the SO is shipped with the value in Qty to Ship.

I tried writing the following code on ExtendedSalesLineTable

trigger OnModify()
    var

    begin
        IF rec."Quantity Shipped" <> 0 then begin
            recSalesLine.SetFilter("No.", Rec."No.");
            recSalesLine.SetRange("Line No.", Rec."Line No.");
            recSalesLine.SetFilter("Document No.", Rec."Document No.");
            if recSalesLine.FindFirst() then begin
                "Quantity Remaining" := (Rec.Quantity - Rec."Quantity Shipped") - rec."Reserved Quantity";
                recSalesLine.Modify();
            end;
        end;
    end;

The problem is, I get Not Available in Rec.Quantity Shipped hence it always show me new field Remaining Quantity as 0.