Mid month price averages

[Marcusneedshelp]

I've run into a bit of trouble with some excel work so I thought id post here to see if u would be able to help me . I already managed to get a working formula to generate monthly prices based on weekly prices. Now i need help because some of the customers require middle of the month price averages and so have averages drawn from the middle of the month(week of the 15th/16th) to the middle of the next month. Is there a way to alter the formula to work from the middle of the months rather than than their start?

I've added an excel sheet with the relevant information and added some dummy data in a similar range to what I'm working with.

rgrds,
Marcus