A unit contacted in enfilade can not have a charge bonus against the enfilading unit.
A curved line of troops (whether infantry, cavalry, or a line of guns) can not be enfiladed in the centre of the line. You can only enfilade the line at the ends.
Another way to check for 10 degrees is by measurement. Project a line from the target counter, parallel to the front of the target counter, to a position adjacent to the firing unit. Measure this distance and call it Y. Measure from this line to the firing unit and call this X. If x divided by Y is .170 or less then the angle is less than 10 degrees.