Looking back through cells in a column to find Specific Text(s)

Good morning,

In the attached sheet I need to check on the Destination in column B, and if it matches set criteria, divide the figure in Column E by 2.

eg If LAX is the destination, then for any rows directly under LAX until the next destination appears, column E needs to equal column D divided by 2.

If the Destination is not LAX, then Column E equals Column D.

the difficulty I am having is that there can be a variety of row numbers for each destination. I am having trouble creating a formula that can basically work backwards from Row-1 in column B until it gets to a text string, and check that text string against the criteria. so it may be row -1 up to about row -9 dependant on the number of flight times available to each destination throughout the week.

Hope you can assist, and thanks in advance

Darren