Hi,
If you use the afni_proc.py it will set up all the commands for you and it will take care of the percent signal change conversions. I highly recommend that you use it in order to process your time series data. afni_proc.py will output a tcsh script with all the commands easily readable so you will see exactly what is being done.
There is plenty of documentation provided with afni_proc.py -help and some course material [
afni.nimh.nih.gov]
cheers,
ziad