Most graphing calculators have a function called linear regression which can be used to find a precise line of fit, using strict rules. This line of fit is then called the line of best fit or the regression line.
The first step is to enter the data points in the calculator. On a TI calculator, this is done by first pressing the STAT button, and then selecting the option Edit....
This gives a number of columns, marked L, L, L etc.
With the help of the arrow keys, choose where in the lists to fill in the data values. Enter the data points' -values in list L and the corresponding -values in L Values are entered into the fields using the number buttons followed by ENTER.
After entering the values, press the STAT button, then select the menu item CALC to the right.
The option LinReg(ax+b) gives a line of best fit, expressed as a linear function in slope-intercept form.