The process of arriving at an approximate answer to a computation such as 0.79 times 403 is called

The process is called estimation
In estimation, we will do a rough calculation of value, quantity, or number.
In most cases, estimation is used if the detail of smaller value is not really mattered that much. In decimal calculation, the most important number is the first number behind the point
So when calculating  0.79 times 403 , most people will make a rough calculation by changing 0.79 to 0.8 instead because the difference wouldn't be matter that much