1. get the mathematical midpoint of the two points (or mean if the node has more than 2 children)
2. subtract the midpoint from 180, or if the midpoint is negative subtract the absolute value of the midpoint
3. if the original midpoint was negative leave the value from 2 as is, else change the sign (if the midpoint is positive the new midpoint is (180-midpoint)*-1
I only use this method if one of the decedents of a node has a different sign than the others and is less than -90 or between 90 and 180. Otherwise I just use the regular midpoint. If a node has more than 2 children I use the mean rather than midpoint.