Change cell reference in formulas based on row condition

I have a spreadsheet that calculates distances between 2 points within groups of data.

The first row in each group will contain a blank cell signifying the starting point of the new data group.

The rest of the rows beneath it will reference the lat, long coordinates on that header row when performing the distance calculation.

The problem is, that I have to repeat this several times, and the only way to tell when to change the reference header (starting point of a new group) is when there is a blank in column B.

So, every time a blank is encountered, the absolute references in my formula would need to change to reflect distance calculations against the new header site.

I've attached a sample workbook which illustrates this, and I am presently updating the absolute cell references manually, but would like to have a formula that can detect a change (null value in Col B) and update the new reference cell accordingly in the formula.

Sufficed to say, I would really appreciate some help on this!