Correct me if I am wrong.
Well, the computation of q is based on the harmonic mean of the samples (taken pairwise) i.e.
q=(YBari-YBarj)/Sqrt[MSw/n*] where n*=2/(1/ni + 1/nj) and MSw is the Mean Square Within (or Error) from the overall ANOVA analysis. Note that YBari and YBarj are the means of group i and group j.
This is as the Tukey-Kramer post hoc test.
As to Scheffe test it is:
Fcv=(k-1)F_alpa-rate(k-1,dfw), where the alpha-rate is, say .05, and dfw is the degrees of freedom within (error) from the overall ANOVA summary.
Note that the Scheffe test is a conservative test and I would not use this.
Last Part: If you give me your degrees of freedom within (dfw), I can provide you with the critical value.
Note: If you use SPSS for k=13 groups then you should have any trouble doing this.