abc parameters for 3dClustSim

Hello AFNI

Im working with an emotional processing fMRI task. I am trying to run the 3dClustSim with the -acf option and the abc parameters to calculate my cluster threshold.

I already calculate the clusters sizes for 2 of my conditions (anger and fear), but I also want to calculate the anger+fear condition.

I am not sure how to calculate the ERRTS of this condition (because I don’t really have a full model in this contrast) to be able to run 3dFWHM and obtain the abc parameters.

the betas of this condition (anger+fear) were calculated mixing the betas of the anger and fear contrast
3dcalc -prefix anger_fear_mean_gr_neutral_Deconvolve.nii -a Beta_anger_5mm.nii -b Beta_fear_5mm.nii -c Beta_neutral_5mm.nii -expr ‘(a+b)/2-c’

Can I average the abc parameters of anger and abc parameters of fear to create the abc parameters of anger+fear?

Thank you for your help