Waterfall Chart Data Labels

[nickFe]

Hi All,
I have modified a friend's waterfall chart so that it can basically be used with any data positive or negative.
The main problem im having is data labels - I want them in the middle of the bars.
If I change the data, especially when it crosses the 0 line, my data labels all require manual manipulation.
Can anyone think of a good way to have the data labels centred in the bars?
I have attached a spreadsheet to have a play with.
Many thanks,
Nick