help with script to copy data from one field to another

Help!  I don't know Javascript, although am willing to learn it.  I have a situation where we created a custom field in the Expense form that calculates the mileage reimbursement based on  the miles traveled and the IRS standard reimbursement rate.  For this line item of the expense record, I need to enter the mileage amount (miles * rate) into the Amount field of the expense (amount field is Netsuite field).  This needs to be done during the entry process, as the Amount field is required to save the Expense record, but because the mileage amount is a calculated field,