Most transactions I enter are
Cash: -X
Taxes:payed: X19%
Expenses:ABC19%: X81%

or
Bank: X
Taxes:received: -X19%
Income:jobA19%: -X81%

having to use the calculator for every single line on your account statement and every piece of paper taxes twice as much time as entering that number and the first characters of the description.
I could do my monthly tax statements in 1/3 the time if it would remember Expenses:ABC19% and Income:jobA19% to be linked to a given tax value.