OK, you are correct, somewhere along the way I lost the sheet reference so formula should be
=IF(ISERR(G8+H8+I8+J8),"",ROUND(SUMPRODUCT(G8:J8,TRANSPOSE(Grade%!C2:C5))/100,0))
although as TRANSPOSE forces you to use CSE you might as well switch to SUM, i.e.
=IF(ISERR(G8+H8+I8+J8),"",ROUND(SUM(G8:J8*TRANSPOSE(Grade%!C2:C5))/100,0))
both still need CTRL+SHIFT+ENTER
regards, barry