In our old ERP system we could do some basic math in every quantity (int, float) or currency field.
For example your supplier sends you a quotation for 125 pieces at a total price of 531.25 USD. Instead of using a calculator and writing 531.25 / 125 = 4.25 and then copying and pasting and typing this into the rate field we could just go to that rate field and type “531.25/125” and it would update to 4.25.
When in a quantity or rate field allow calculations with basic math (addition subtraction multiplication and division): 2+1, 2-1, 2*2, 2/2. When the calculation is done show a grey infobox on the bottom showing the math which then fades away.