How can i make discount allowed calculated on the face of the summary immediately discount is checked and percentage assigned to a particular invoice.
Your question is not clear. What do you mean by “on the face of the summary?” Did you mean a sales invoice rather than the summary?
You can set a default discount percentage under Form Defaults in Settings. See https://www.manager.io/guides/14059. This discount will be applied to the first line item of every sales invoice. Subsequent line items will need to be edited as you create the sales invoice.