Or maybe

=IFERROR(IF(COUNTBLANK(B6:B35)>29,"N/A",(SUMPRODUCT(--(ISNUMBER(MATCH('Jun 08'!$A$2:$A$731,$B$6:$B$35,0))),'Jun 08'!$B$2:$B$731)/(SUMPRODUCT(--(ISNUMBER(MATCH('Jun 08'!$A$2:$A$731,$B$6:$B$35,0))),'Jun 08'!$C$2:$C$731)*2))*100),"N/A")