how to lookup value from get summary in related table

[Wahyu]

I have 2 table: Test7 and Test8 where related with ItemNumber.

Test7 fields: ItemNumber,Part,Qty,Minutes,SumMinutes, SumQty,,Total7

Total calculation:

GetSummary ( SumMinute;ItemNumber ) * GetSummary ( SumQty;ItemNumber )

Also sortsummary by ItemNumber.

Test8 fields: ItemNumber, Total8, Used, TotalUsed (is calculation =Total8*Used)

My question is, how Test8:Total8 lookup value from Test7:Total7

Thank you

note: how to upload attchment from my PC to this case.