VMG (velocity made good)is not an angle. It is the velocity in the direction of the waypoint currently being used. Pretty sure there is not a calibration as it uses the GPS SOG and COG to calculate it.
SO..... I'm thinking you are using the wrong waypoint. You should choose one that is centered over the destination you which to reach or an intermediate destination if there is a curve in the river etc.
VMG = SOG*sin(theta) where theta is the angle between the boat COG and the direction to the waypoint being used.