You can use a workflow to update the line items in realtime as you are entering lines using after field edit trigger. Assuming your transaction has the items Sublist. However you cannot perform calculations in realtime. You can source values but you can’t calculate values from different fields. To do calculations it requires a server side trigger with workflows like before submit or after submit. Not sure why but I suspect the “calculator” NetSuite uses in formulas relies on server data so a server trigger is required
Alternatively you could do this with a client script of you aren’t using the items Sublist or really need it in “realtime” while editing the line