The critical F-statistic is the point at which the F-statistic has a proportion in the tails of the distribution equal to the decision level of the test. Usually, these figures are referred to in tables. Such a table can be found in the Index of Applied Linear Statistical Models, Fourth Edition, by Neter, Kutner, Wasserman, and Nachtsheim.
Here we will use a significance level of 0.05, and find the critical F-statistic for 8 df in the numerator and 33 df in the denominator. These figures come from the example of Teacher Pay in the analysis of covariance in the model with the included interactions. We will derive this graphically from the probability distribution plot and from the probability distributions from the Calc menu.