How to make the BB8 robot turn a perfect 90 degrees

In the ROS service quiz solution, they used an angular velocity of 0.2 to make the robot form the square. After studying the simulation of the robot, it did not move in a perfect square, It moved in angle that seems like 120 degrees. Please how can I make the robot move a perfect 90 degrees.

After experimenting with the simulation, I realised that setting angular veclocity of 0.4 make the bb8 robot turn a perfect 90 degrees.