If the Earth were to have an atmosphere identical to Venus, how much would the average temperature increase

Earth is about 1.39 times farther from the Sun than Venus.
Energy received from the Sun is inversely proportional to the square of the distance.
So the Earth receives 1/1.39^2 = 0.51 as much energy. (about half).
The temperature would be about half of Venus (using kelvins)

The surface of Venus is about 733 kelvin

Earth would be 374 kelvin. This is 101 degrees Celsius. (just over the boiling point of water)