%or mod has no standard behavior across programming languages. Some, like js, will will keep the sign of the original number, and some will wrap around 0 to the positive number. Which I think would be more useful.
Anyways since angle() gives an angle -180 to 180 you can convert it to the 0-360 range with
(a%360+360)%360
/// the second % is to get it back into the 0-360 range.
Or
A<0?360+A:A
Alternately you could use the angle conditions or the angleDiff expression to check your angles. They can be helpful especially when you have to deal with the 0-360 seam