Re-computation of Grand Total / Net total after change in another custom discount field

I want to apply the same trigger calculation whenever we change the discount_amount field value.
I have another field as discount_amount_new
now whenever i change or set the value in the field then automatically the grand total/taxes/net amount, all these calculations should happen through custom script.

Is there any custom standard function which I m able to see like calculate() or recalculate() or calculate_taxes …

please help me out in solving the problem

Please Help!

You may try cur_frm.refresh(); in the custom script.