Correct me if I am wrong.

Well, the computation ofqis 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.