Omit blank cells from SUMPRODUCT average formula?

I'm using the following formula to get an average of specific percentages matching specific cells in a specific column

=SUMPRODUCT(($B$2:$B$169="Extra")*((AA$2:AA$169)/COUNTIF($B$2:$B$169,"=Extra")))

i.e under each monthly column this formula displays the average for all 'Extra' services that month.

Column A = List of sites
Column B = Standard, Premium, Extra
Columns C to X = Percentages for that month

Site : Service : May 09 : June 09 : July 09
SITE A : Standard : 100% : 90.99% : 95.34%
SITE A : Premium : 100% : [Blank] : 95.34%
SITE A : Extra : 95.55% : 90.99% : 95.34%
SITE B : Standard : 100% : 90.99% : [Blank]
SITE B : Premium : 100% : 90.99% : 95.34%
SITE B : Extra : 95.55% : [Blank] : 93.34%
SITE C : Standard : 100% : 90.99% : 95.34%
SITE C : Premium : [Blank] : 90.99% : 95.24%
SITE C : Extra : 95.55% : 90.99% : 95.34%

The formula is working perfectly except when the percentage column has a blank cell, which returns an incorrect result.

Is there a way to adapt this formula so that blank cells are ignored data range of percentages.

Please note I don't want to change the order of the rows in the table.

Thanks in advance!