Tapping the amount field does not bring up the system number pad. It opens a keypad with the digits, a decimal point, four operators (+, -, ×, ÷), equals, backspace, and a cancel control.
The keypad shows two lines: the expression you are building up top, and the running result of that expression underneath. Both update with every keypress, so 85.40 - 4.50 - 6.20 resolves to 74.70 in one continuous motion without leaving the form.
Equals closes the keypad and writes the number into the amount field; the expression itself is not saved. Cancel or dismissing the keypad leaves the field untouched.
The point is removing the round trip through the system calculator - no app switch, no clipboard dance, no mental arithmetic to second-guess at the end of a long evening.