It's hard to beat the built-in calculator functionality found in some accounting software. For example, in Quickbooks, in any numeric input field, you can type

It's hard to beat the built-in calculator functionality found in some accounting software. For example, in Quickbooks, in any numeric input field, you can type a number and then an operator (like +, / etc) and it automatically shifts into calculator mode right there in the input field, with the result pasted automatically into the field. Similarly, my income tax software pops up a built-in calculator upon pressing a function key, and the result is automatically pasted back when done.