(sorry, deleted earlier reply where I messed it up quite badly!)
You want the 'y' value as the first argument to the ATAN2 function. This returns radians so you want to convert to degrees. Then if you want a bearing you will want to return this normalised to 0..360 instead of -180..180. I've used your workbook's measure names in the below.
DEGREES(ATAN2(SIN([BH Longitude]-[Longitude])*COS([BH Latitude]),COS([Latitude])*SIN([BH Latitude])-SIN([Latitude])*COS([BH Latitude])*COS([BH Longitude]-[Longitude])))
IF [Bearing1] < 0 THEN
360 + [Bearing1]