Hi Overwing,
I've just checked it and it works okay, to within a degree or so. I checked it with a EGBB Cowly SID by plugging the Honiley and Cowly co-ordinates in.
There are three possible causes for the grief you might be getting. Firstly, the formula is a trigonometric great circle calculation therefore the result will be in degs True. This will account for small errors. Secondly, the author is from the States and therefore he treats western hemisphere longitudes as positive, whereas we Brits (and Irish?) would probably use a positive east and negative west. This would create somewhat large errors!
Finally, Excel is really picky with brackets. Depending on the arithmetic functions involved, you often need more than you would to write the formula.
For your viewing please I have created a small Excel workbook which works okay, I've even reversed the numbering convention for east/west co-ordinates to the 'correct' way (i.e. +ve E and N, -ve W and S). As I said I've checked that it works and, after applying the variation stated at the top of the instrument departure, the result was only a degree out. This should not be a big deal if you are only flying legs of less than 100nm.
Let me know if you want the spreadsheet emailing, you might have got it working yourself by now.
Cheers
Charley