You are right. There is no way we can refer variables/calculated fields defined in a list form in the workflow associated with the list.
But, we need a way to pass/refer the form values in workflow. I am working on a leave application, where user wants to edit the leave begin/end date. All changes needs to go in approval process. I customized the form to show date/time controls for new begin and end date. I want to pass the new values to workflow. Once the Approver approves them, then only it should reflect the actual list item. Is there a way I can pass this new date/time values from List Form to List Workflow without storing them the list.