For traders, eyeball it. Manufacturers should probably spreadsheet it, otherwise it's difficult to tell if you're making a profit or not.
It depends on my volume and profit margins tbh. If i am shifting a load of cheap modules with little profit margin then yes i will just eye ball it, but with more valuable cargo then i will spreadsheet it. Although everything i sell ends up on a spreadsheet to compare monthly growth. but its rare i do it on a per item basis