{=AVERAGE(IF((MOD(ROW(CLM10000!C3:C646),7)=3)*(C3: C646<""),C3:C646))}

I have this entered in cell Adjust PtsC1. As I understand it it gives the
average of a column using the data in every 7th row, starting with the 3rd
row. It seems to work well, but my problem is when I try to drag it down
(even with Ctrl+Drag).

I've changed the formula
{=AVERAGE(IF((MOD(ROW(CLM10000!C3:C646),7)=4)*(C3: C646<""),C3:C646))}, so it
should use every 7th row starting with the 4th row. Is there a trick to
copying, pasting or moving Array formulas?

Any help? Thanks!