Find last previous non blank value and summarise previous 6 months

Please see attached file. Need to find latest non blank value - in attached file it is highlighted in yellow. From there, want to summarise 26 weeks back so, in the attached file:

Row 2 would be finding 750 and summarised back 26 weeks from 30 sep 2012
Row 3 would be finding 2250 and summarised back 26 weeks from 2 dec 2012
Row 4 would be finding 5000 and summarised back 26 weeks from 4 nov 2012

Thanks in advance
Shaz