> so the calculation basically works out to be 2 * atan2 (0.666666666666666.....)

atan2 takes 2 parameters, this is important and likely where you've gone 
wrong. Make sure you've got the parameters the right way round - Excel 
takes (x,y) not (y,x).

If you've already pre-calculated the y/x gradient (eg 0.66666) then for 
atan2 to give the correct result in Excel you need to use 
atan2(1,0.666666). If you do it the other way round atan2(0.66666,1) 
then you're going to get the wrong answer.

>   converted to degrees
> which ought to be very close to 67.38

2*degrees(atan2(1,0.666666)) gives 67.38 degrees in Excel.

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