Alternative to SUMPRODUCT???

The formula below appears hundereds of times in my xls and I have a feeling it is bogging down my system (the xls's that use this formula range from 15-50 MB).

=SUMPRODUCT('Hours Per Unit'!$D$10:OFFSET('Hours Per Unit'!$D$10,ROW($H$62)-6,0),O$6:O$62)-O123

Any ideas of a more efficient way to write this would be much appreciated.

Or if you think this is the best -- let me know too. Thanks!